A bag contains 5 red marbles and some white marbles. If the probability of drawing a white marble is double that of red marble, the number of white marbles in the bag is

The correct answer is (b).

Let $x$ be the number of white marbles in the bag. The probability of drawing a white marble is $\frac{x}{x+5}$. The probability of drawing a red marble is $\frac{5}{x+5}$. We are given that the probability of drawing a white marble is double that of red marble, so $\frac{x}{x+5} = 2 \cdot \frac{5}{x+5}$. This simplifies to $x = 5$.

Option (a) is incorrect because there are only 5 red marbles in the bag, so there cannot be 10 white marbles. Option (c) is incorrect because there are only 5 red marbles in the bag, so there cannot be 20 white marbles. Option (d) is incorrect because the number of white marbles is not 15.