Discussion of Question with ID = 097 under Probability

A box contains 20 red balls and an unknown number of blue balls. If the probability of drawing a blue ball is 5 times the probability of drawing a red ball, then how many blue balls are there in the box?

Let the number of blue balls be x. Chances of drawing a blue ball are $x/{x + 20}$ = 5 × $20/{x + 20}$, which gives x = 5 × 20 = 100.