Question

An urn contains 9 red balls and x green balls. If the probability of picking a red ball is thrice that of picking a green ball, then x = ______.

Answer 1

An urn contains 9 red balls and x green balls. If the probability of picking a red ball is thrice that of picking a green ball, then x = 3.

Explanation:

Equate the probabilities: \[\frac{9}{9+x}=3\times\frac{x}{9+x}\] 

Hence, 9 = 3x, so the answer is 3.