A bag contains 5 red balls and some blue balls. If the probability of drawing a blue ball is double that of a red ball, determine the number of blue balls in the bag.

The correct answer is 10

Let the number of blue balls in the bag be x.
∴ Total balls in the bag = red balls + blue balls = 5 + x
Number of possible outcomes = 5 + x
Let E denote the outcome where the ball drawn is red.
∴ Probability of event E = P(E) = $\frac{5}{(5 + x)}$

Probability of event of drawing a blue ball = P(F) = $\frac{x}{(5 + x)}$
But since P(F) = 2 * P(E),
$\frac{x}{(5 + x)}$ = 2 * $\frac{5}{(5 + x)}$
or x = 2 * 5 = 10