How many three-digit numbers are possible such that the difference between the original number and the number obtained by reversing the digits is 396? (no digit is repeated)
4
5
50
40
Answer is Right!
Answer is Wrong!
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UPSC CAPF – 2023
Let the three-digit number be 100a + 10b + c, where a is a digit from 1 to 9, and b and c are digits from 0 to 9. The digits a, b, and c must be distinct.
The number obtained by reversing the digits is 100c + 10b + a.
The difference between the original number and the reversed number is given as 396.
(100a + 10b + c) – (100c + 10b + a) = 396
99a – 99c = 396
99(a – c) = 396
a – c = 396 / 99 = 4.
We need to find the number of triplets (a, b, c) such that:
1. a is a digit from 1 to 9.
2. c is a digit from 0 to 9.
3. b is a digit from 0 to 9.
4. a, b, c are distinct (a != b, b != c, a != c).
5. a – c = 4.
Since a – c = 4 and a is a single digit, a > c, which guarantees a != c. Also, since a >= 1, c >= 0.
Let’s list the possible pairs of (a, c) where a – c = 4 and a is in {1..9}, c is in {0..9}:
– If c = 0, a = 4. Pair (4, 0).
– If c = 1, a = 5. Pair (5, 1).
– If c = 2, a = 6. Pair (6, 2).
– If c = 3, a = 7. Pair (7, 3).
– If c = 4, a = 8. Pair (8, 4).
– If c = 5, a = 9. Pair (9, 5).
There are 6 such pairs for (a, c).
For each pair (a, c), the digit b must be distinct from a and c. There are 10 possible digits (0-9). Since a and c are distinct and are already chosen, b can be any of the remaining 10 – 2 = 8 digits.
If we strictly follow N – N_rev = 396, there are 6 * 8 = 48 such numbers. However, 48 is not among the options.
Let’s consider a common convention in such problems: the reversed number must also be a three-digit number. This implies that the units digit of the original number, c, cannot be 0.
If c must be in {1..9} (and a in {1..9}) with a-c=4:
– If c = 1, a = 5. Pair (5, 1).
– If c = 2, a = 6. Pair (6, 2).
– If c = 3, a = 7. Pair (7, 3).
– If c = 4, a = 8. Pair (8, 4).
– If c = 5, a = 9. Pair (9, 5).
There are 5 such pairs for (a, c) if c!=0.
For each of these 5 pairs, b must be distinct from a and c. There are 10 – 2 = 8 possible digits for b.
Total number of such three-digit numbers = 5 pairs * 8 options for b per pair = 40.
This matches option D. This suggests the implicit condition that the reversed number is also a three-digit number (c!=0) was intended.