UPSC CAPF Archives

21. A and B start at the same time to reach the same destination. B travel

A and B start at the same time to reach the same destination. B travelled at $\frac{5}{7}$ of A’s speed and reached the destination 1 hour 20 minutes after A. What was the time taken by B to reach the destination ?

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4 hours 40 minutes

4 hours 55 minutes

5 hours 5 minutes

5 hours 15 minutes

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Answer is Wrong!

This question was previously asked in

UPSC CAPF – 2024

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The time taken by B to reach the destination was 4 hours 40 minutes.

Let the distance to the destination be D.
Let A’s speed be $V_A$ and B’s speed be $V_B$.
We are given that B travelled at $\frac{5}{7}$ of A’s speed, so $V_B = \frac{5}{7} V_A$.
The time taken by A to reach the destination is $T_A = \frac{D}{V_A}$.
The time taken by B to reach the destination is $T_B = \frac{D}{V_B}$.
Substitute $V_B = \frac{5}{7} V_A$ into the expression for $T_B$:
$T_B = \frac{D}{(5/7)V_A} = \frac{7D}{5V_A} = \frac{7}{5} \left(\frac{D}{V_A}\right) = \frac{7}{5} T_A$.
We are given that B reached the destination 1 hour 20 minutes after A.
1 hour 20 minutes $= 1 + \frac{20}{60}$ hours $= 1 + \frac{1}{3}$ hours $= \frac{4}{3}$ hours.
The difference in time is $T_B – T_A = \frac{4}{3}$ hours.
Substitute $T_B = \frac{7}{5} T_A$:
$\frac{7}{5} T_A – T_A = \frac{4}{3}$.
$(\frac{7}{5} – 1) T_A = \frac{4}{3}$.
$(\frac{7-5}{5}) T_A = \frac{4}{3}$.
$\frac{2}{5} T_A = \frac{4}{3}$.
Now solve for $T_A$:
$T_A = \frac{4}{3} \times \frac{5}{2} = \frac{20}{6} = \frac{10}{3}$ hours.
We need to find $T_B$:
$T_B = \frac{7}{5} T_A = \frac{7}{5} \times \frac{10}{3} = \frac{7 \times 2}{3} = \frac{14}{3}$ hours.
Convert $\frac{14}{3}$ hours into hours and minutes.
$\frac{14}{3} = 4 \frac{2}{3}$ hours.
$\frac{2}{3}$ hours $= \frac{2}{3} \times 60$ minutes $= 40$ minutes.
So, $T_B = 4$ hours 40 minutes.

For a fixed distance, speed is inversely proportional to time. If $V_B = k V_A$, then $T_B = \frac{1}{k} T_A$. Here, $k = 5/7$, so $T_B = \frac{1}{5/7} T_A = \frac{7}{5} T_A$. The difference in times is $T_B – T_A = (\frac{7}{5} – 1)T_A = \frac{2}{5} T_A$. Given this difference, we can find $T_A$ and subsequently $T_B$.

22. Two taps A and B can fill a tank with water in 20 minutes and 15 minut

Two taps A and B can fill a tank with water in 20 minutes and 15 minutes respectively. A third tap C can empty the full tank in 6 minutes. After the taps A and B fill the tank for 5 minutes, the tap C is opened to empty while A and B continue to fill it. In how many minutes after the start shall the tank get empty ?

$10rac{1}{3}$ minutes

$10rac{2}{5}$ minutes

$10rac{4}{5}$ minutes

$11rac{2}{3}$ minutes

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This question was previously asked in

UPSC CAPF – 2024

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The tank shall get empty $11\frac{2}{3}$ minutes after tap C is opened. (Assuming this is the intended meaning based on options).

Tap A fills the tank in 20 minutes, so its filling rate is $\frac{1}{20}$ tank per minute.
Tap B fills the tank in 15 minutes, so its filling rate is $\frac{1}{15}$ tank per minute.
Tap C empties the tank in 6 minutes, so its emptying rate is $\frac{1}{6}$ tank per minute. (This is a negative rate from the perspective of filling).

Initially, taps A and B fill the tank for 5 minutes.
Combined filling rate of A and B = $\frac{1}{20} + \frac{1}{15}$.
LCM of 20 and 15 is 60.
Combined rate of A and B = $\frac{3}{60} + \frac{4}{60} = \frac{7}{60}$ tank per minute.
Volume filled in the first 5 minutes = $5 \times \frac{7}{60} = \frac{35}{60} = \frac{7}{12}$ of the tank.

After 5 minutes, tap C is opened, while A and B continue to fill. All three taps are now open.
The net rate of change in tank volume is the sum of individual rates:
Net rate = Rate of A + Rate of B + Rate of C
Net rate = $\frac{1}{20} + \frac{1}{15} – \frac{1}{6}$.
LCM of 20, 15, and 6 is 60.
Net rate = $\frac{3}{60} + \frac{4}{60} – \frac{10}{60} = \frac{3+4-10}{60} = \frac{-3}{60} = -\frac{1}{20}$ tank per minute.
The net rate is negative, which means the tank is emptying at a rate of $\frac{1}{20}$ tank per minute.

At the moment C is opened, the tank is $\frac{7}{12}$ full. Since the net rate is negative, the tank will empty from this level.
The time taken to empty the tank from this point is the current volume divided by the emptying rate:
Time to empty = $\frac{\text{Current Volume}}{\text{Net Emptying Rate}} = \frac{7/12}{1/20}$ (using the absolute value of the negative rate).
Time to empty = $\frac{7}{12} \times 20 = \frac{140}{12} = \frac{35}{3}$ minutes.

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The question asks “In how many minutes after the start shall the tank get empty ?”. This phrasing suggests the total time from t=0.
Total time from start = Time A and B filled alone + Time A, B, C worked together until empty.
Total time = 5 minutes + $\frac{35}{3}$ minutes = $\frac{15}{3} + \frac{35}{3} = \frac{50}{3}$ minutes.
$\frac{50}{3} = 16 \frac{2}{3}$ minutes.

However, $16 \frac{2}{3}$ minutes is not among the options. The value $\frac{35}{3}$ minutes $= 11 \frac{2}{3}$ minutes is option D. This strongly suggests that the question is asking for the time elapsed *after tap C is opened* until the tank is empty, rather than the total time from the very start. Based on the options, we interpret the question as asking for the time after C is opened.
Time after C is opened = $\frac{35}{3}$ minutes = $11\frac{2}{3}$ minutes.

The key is calculating the net rate when all taps are open. A positive net rate means filling, a negative net rate means emptying. The volume to be emptied is the volume present in the tank when the emptying process starts. The ambiguity in the question phrasing highlights the importance of checking options in MCQs; often, one calculation leads to an answer in the options while another plausible interpretation does not.

23. A, B and C can finish a work in 20, 25 and 30 days, respectively. They

A, B and C can finish a work in 20, 25 and 30 days, respectively. They start working together but B quits after working for 3 days. In how many days from the start shall the work be completed ?

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$9rac{8}{15}$ days

$10rac{1}{15}$ days

$10rac{14}{25}$ days

$11rac{1}{10}$ days

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Answer is Wrong!

This question was previously asked in

UPSC CAPF – 2024

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The work shall be completed in $10\frac{14}{25}$ days from the start.

Let the total work be 1 unit.
A can finish the work in 20 days, so A’s daily work rate is $\frac{1}{20}$.
B can finish the work in 25 days, so B’s daily work rate is $\frac{1}{25}$.
C can finish the work in 30 days, so C’s daily work rate is $\frac{1}{30}$.
They start working together (A, B, and C). Their combined daily work rate is $\frac{1}{20} + \frac{1}{25} + \frac{1}{30}$.
To add these fractions, find a common denominator (LCM of 20, 25, 30 is 300):
Combined rate = $\frac{15}{300} + \frac{12}{300} + \frac{10}{300} = \frac{15+12+10}{300} = \frac{37}{300}$.
They work together for 3 days. Work done in the first 3 days = $3 \times \frac{37}{300} = \frac{37}{100}$.
After 3 days, B quits. The remaining work is $1 – \frac{37}{100} = \frac{63}{100}$.
The remaining work is done by A and C. Their combined daily work rate is $\frac{1}{20} + \frac{1}{30}$.
LCM of 20 and 30 is 60:
Combined rate of A and C = $\frac{3}{60} + \frac{2}{60} = \frac{5}{60} = \frac{1}{12}$.
Time taken by A and C to finish the remaining work = $\frac{\text{Remaining Work}}{\text{Combined Rate of A and C}} = \frac{63/100}{1/12}$.
Time taken = $\frac{63}{100} \times 12 = \frac{63 \times 3}{25} = \frac{189}{25}$ days.
The question asks for the total number of days from the start.
Total time = Time A, B, C worked together + Time A and C worked together.
Total time = 3 days + $\frac{189}{25}$ days.
Total time = $\frac{3 \times 25}{25} + \frac{189}{25} = \frac{75+189}{25} = \frac{264}{25}$ days.
Converting the improper fraction to a mixed number: $264 \div 25$. $264 = 10 \times 25 + 14$.
So, $\frac{264}{25} = 10\frac{14}{25}$ days.

The work done is proportional to the rate of work and the time spent. The total work is assumed to be 1 unit or the LCM of the individual times can be taken as the total units of work. Here, LCM of 20, 25, 30 is 300 units.
A’s daily units: 300/20 = 15 units/day.
B’s daily units: 300/25 = 12 units/day.
C’s daily units: 300/30 = 10 units/day.
In the first 3 days, (A+B+C) work = (15+12+10) * 3 = 37 * 3 = 111 units.
Remaining work = 300 – 111 = 189 units.
Remaining work is done by A and C. Their combined rate = 15 + 10 = 25 units/day.
Time taken for remaining work = 189 units / 25 units/day = $\frac{189}{25}$ days.
Total time = 3 days + $\frac{189}{25}$ days = $3 + 7\frac{14}{25} = 10\frac{14}{25}$ days.

24. A shopkeeper sold a product at 30% loss. Had his selling price been ₹1

A shopkeeper sold a product at 30% loss. Had his selling price been ₹150 more, he would have made a profit of 10%. What was the cost price ?

₹375

₹400

₹425

₹450

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UPSC CAPF – 2024

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The cost price of the product was ₹375.

Let CP be the cost price of the product.
Initially, the shopkeeper sold the product at a 30% loss.
Selling Price 1 (SP1) = CP – 30% of CP = CP – 0.30 * CP = 0.70 * CP.
If the selling price had been ₹150 more, he would have made a profit of 10%.
Selling Price 2 (SP2) = SP1 + ₹150.
Also, SP2 represents a 10% profit on the cost price.
SP2 = CP + 10% of CP = CP + 0.10 * CP = 1.10 * CP.
So, we have the equation:
0.70 * CP + 150 = 1.10 * CP.
Subtract 0.70 * CP from both sides:
150 = 1.10 * CP – 0.70 * CP
150 = 0.40 * CP.
To find CP, divide 150 by 0.40:
CP = $\frac{150}{0.40} = \frac{150}{\frac{4}{10}} = \frac{150 \times 10}{4} = \frac{1500}{4}$.
CP = 375.

The difference between the two selling prices (SP2 – SP1) is equal to the difference between the profit/loss percentages applied to the cost price.
SP2 – SP1 = ₹150.
SP2 = CP + 10% profit = 1.10 CP.
SP1 = CP – 30% loss = 0.70 CP.
SP2 – SP1 = 1.10 CP – 0.70 CP = 0.40 CP.
So, 0.40 CP = 150.
CP = 150 / 0.40 = 375.

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25. If $x+\frac{1}{x}=2$, then which one of the following is the value of

If $x+\frac{1}{x}=2$, then which one of the following is the value of $x^{32}+\frac{1}{x^{32}}$ ?

-1

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This question was previously asked in

UPSC CAPF – 2024

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The correct answer is 2.

Given the equation $x + \frac{1}{x} = 2$. To find the value of $x$, we can multiply the entire equation by $x$ (assuming $x \neq 0$, which must be true for $1/x$ to be defined).
$x(x + \frac{1}{x}) = 2x$
$x^2 + 1 = 2x$
Rearranging the terms gives a quadratic equation:
$x^2 – 2x + 1 = 0$
This equation is a perfect square trinomial, which can be factored as $(x-1)^2 = 0$.
Solving for $x$, we get $x-1 = 0$, which means $x = 1$.
Now, we need to find the value of the expression $x^{32} + \frac{1}{x^{32}}$.
Substitute $x = 1$ into the expression:
$1^{32} + \frac{1}{1^{32}} = 1 + \frac{1}{1} = 1 + 1 = 2$.

This problem utilizes a common algebraic trick where if $x + \frac{1}{x} = 2$, then $x$ must be 1 (for real values of x). If $x + \frac{1}{x} = -2$, then $x$ must be -1. For other values like $x + \frac{1}{x} = 1$ or $-1$, $x$ would be complex numbers. The property $1^n = 1$ for any integer $n$ is used in the final calculation.

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26. The ratio of present ages (in years) of X to Y is equal to the ratio o

The ratio of present ages (in years) of X to Y is equal to the ratio of present ages (in years) of Y to Z. If the present age of Y is 15 years, then which of the following can be the sum of the ages (in years) of X, Y and Z ?

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This question was previously asked in

UPSC CAPF – 2024

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The correct answer is C) 49.

Let the present ages of X, Y, and Z be $X, Y, Z$ years respectively.
The ratio of present ages of X to Y is equal to the ratio of present ages of Y to Z.
$\frac{X}{Y} = \frac{Y}{Z}$
This implies $Y^2 = XZ$.
Given that the present age of Y is 15 years, so $Y=15$.
$15^2 = XZ \implies 225 = XZ$.
We are looking for a possible sum of the ages $X+Y+Z = X+15+Z$. We need to find integer pairs $(X, Z)$ whose product is 225 and check the sum $X+15+Z$ against the options.

We list integer factors of 225 and form pairs $(X, Z)$ such that $XZ=225$. We can assume $X \le Z$ without loss of generality, as the sum $X+15+Z$ will be the same for $(X,Z)$ and $(Z,X)$.
Factors of $225 = 3^2 \times 5^2$: 1, 3, 5, 9, 15, 25, 45, 75, 225.
Possible pairs $(X, Z)$ with $X \le Z$ and $XZ=225$:
– $(1, 225) \implies X+15+Z = 1 + 15 + 225 = 241$. Not an option.
– $(3, 75) \implies X+15+Z = 3 + 15 + 75 = 93$. Not an option.
– $(5, 45) \implies X+15+Z = 5 + 15 + 45 = 65$. Not an option.
– $(9, 25) \implies X+15+Z = 9 + 15 + 25 = 49$. This is option C.
– $(15, 15) \implies X+15+Z = 15 + 15 + 15 = 45$. Not an option. (Note: this case means X=Y=Z=15, which satisfies the ratio condition, although ages are usually assumed different in such problems unless stated).

Since 49 is one of the calculated possible sums (when ages are 9, 15, and 25), it is a valid answer.

27. Which of the following statements is/are correct ? Select the answer u

Which of the following statements is/are correct ?
Select the answer using the code given below:

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1. The average of four numbers 10, 15, 20 and 25 is 17.5
2. If a, b and c are three different natural numbers such that a + b + c = abc, then the average of a, b and c is 3

1 only

2 only

Both 1 and 2

Neither 1 nor 2

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This question was previously asked in

UPSC CAPF – 2024

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The correct answer is A) 1 only.

Let’s evaluate each statement:
1. The average of four numbers 10, 15, 20 and 25:
Sum $= 10 + 15 + 20 + 25 = 70$. Number of terms $= 4$. Average $= \frac{70}{4} = 17.5$. Statement 1 is correct.
2. If a, b and c are three different natural numbers such that a + b + c = abc, then the average of a, b and c is 3.
The average of a, b, and c is $\frac{a+b+c}{3}$. Given $a+b+c = abc$, the average is $\frac{abc}{3}$. We need to find if $\frac{abc}{3}$ is always 3 for different natural numbers a, b, c satisfying $a+b+c=abc$.
We look for solutions to $a+b+c = abc$ in different natural numbers. Assume $1 \le a < b < c$. If $a=1$, $1+b+c=bc \implies bc-b-c=1 \implies (b-1)(c-1)-1=1 \implies (b-1)(c-1)=2$. Since $1 < b < c$, we have $b-1 < c-1$. The only integer factors of 2 are (1, 2). So $b-1=1$ and $c-1=2$, which gives $b=2$ and $c=3$. The set {1, 2, 3} satisfies $1+2+3=6$ and $1 \times 2 \times 3 = 6$. These are different natural numbers. For this solution set {1, 2, 3}, the average is $\frac{1+2+3}{3} = \frac{6}{3} = 2$.

We showed in the thought process that {1, 2, 3} is the only solution set in different natural numbers. Since the average for this solution is 2, not 3, statement 2 is false.
Therefore, only statement 1 is correct.

28. Which one of the following is the average of first five multiples of e

Which one of the following is the average of first five multiples of each of the numbers from 11, 12, 13, …, 20 ?

40.5

42.5

44.5

46.5

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UPSC CAPF – 2024

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The correct answer is D) 46.5.

We need to find the average of the first five multiples for each number from 11 to 20.
For any number $N$, the first five multiples are $N, 2N, 3N, 4N, 5N$.
The sum of these multiples is $N + 2N + 3N + 4N + 5N = (1+2+3+4+5)N = 15N$.
The average of these five multiples is $\frac{15N}{5} = 3N$.
So, for each number $N$ from 11 to 20, the average of its first five multiples is $3N$.
We need to find the average of the values $3 \times 11, 3 \times 12, …, 3 \times 20$.

The numbers are $33, 36, 39, …, 60$. There are $20 – 11 + 1 = 10$ such numbers.
The average of these 10 numbers is $\frac{(3 \times 11) + (3 \times 12) + … + (3 \times 20)}{10}$.
We can factor out 3 from the numerator: $\frac{3 \times (11 + 12 + … + 20)}{10}$.
The numbers 11, 12, …, 20 form an arithmetic progression. The sum of an arithmetic progression is $\frac{\text{number of terms}}{2} \times (\text{first term} + \text{last term})$.
Sum of $11 + … + 20 = \frac{10}{2} \times (11 + 20) = 5 \times 31 = 155$.
The required average is $\frac{3 \times 155}{10} = \frac{465}{10} = 46.5$.
Alternatively, the average of $3N$ for $N=11, …, 20$ is $3 \times (\text{Average of } 11, …, 20)$. The average of $11, …, 20$ is $\frac{11+20}{2} = 15.5$. The required average is $3 \times 15.5 = 46.5$.

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29. The angle (in degrees) made by a sector having area one-sixth of the a

The angle (in degrees) made by a sector having area one-sixth of the area of a semicircle is

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15°

30°

45°

60°

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This question was previously asked in

UPSC CAPF – 2024

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The correct answer is B) 30°.

Let the radius of the semicircle be $r$. The area of the semicircle is $\frac{1}{2} \pi r^2$.
The area of the sector is given as one-sixth of the area of the semicircle.
Area of sector $= \frac{1}{6} \times \left(\frac{1}{2} \pi r^2\right) = \frac{1}{12} \pi r^2$.
The formula for the area of a sector with angle $\theta$ (in degrees) and radius $r$ is $\frac{\theta}{360°} \pi r^2$.

Equating the two expressions for the area of the sector:
$\frac{\theta}{360°} \pi r^2 = \frac{1}{12} \pi r^2$.
Assuming $r > 0$, we can cancel $\pi r^2$ from both sides:
$\frac{\theta}{360°} = \frac{1}{12}$.
Now, solve for $\theta$:
$\theta = \frac{360°}{12} = 30°$.
The angle made by the sector is 30 degrees.

30. The remainder, when 1 + (1 × 2) + (1 × 2 × 3) + … + (1 × 2 × 3 × …

The remainder, when 1 + (1 × 2) + (1 × 2 × 3) + … + (1 × 2 × 3 × … × 500) is divided by 8, is

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This question was previously asked in

UPSC CAPF – 2024

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The correct answer is A) 1.

We need to find the remainder of the sum $S = 1! + 2! + 3! + … + 500!$ when divided by 8.
Let’s compute the first few factorials modulo 8:
$1! = 1 \equiv 1 \pmod 8$
$2! = 2 \equiv 2 \pmod 8$
$3! = 6 \equiv 6 \pmod 8$
$4! = 24 = 3 \times 8 \equiv 0 \pmod 8$
$5! = 5 \times 4!$. Since $4!$ is a multiple of 8, $5!$ is also a multiple of 8. $5! \equiv 0 \pmod 8$.
In general, for any integer $n \ge 4$, $n!$ includes $4!$ as a factor. Since $4!$ is a multiple of 8, $n!$ is a multiple of 8 for all $n \ge 4$.

So, $n! \pmod 8 = 0$ for $n \ge 4$.
The sum modulo 8 is:
$S \pmod 8 = (1! + 2! + 3! + 4! + … + 500!) \pmod 8$
$S \pmod 8 = (1! \pmod 8 + 2! \pmod 8 + 3! \pmod 8 + 4! \pmod 8 + … + 500! \pmod 8) \pmod 8$
$S \pmod 8 = (1 + 2 + 6 + 0 + 0 + … + 0) \pmod 8$
$S \pmod 8 = (1 + 2 + 6) \pmod 8$
$S \pmod 8 = 9 \pmod 8$
$S \pmod 8 = 1$.
The remainder is 1.