    Author: Rodney Shaghoulian
    Github: github.com/RodneyShag
HackerRank: hackerrank.com/RodneyShag


Answer: 17/42

Urn X has a 4/7 probability of giving a red ball
Urn Y has a 5/9 probability of giving a red ball
Urn Z has a 1/2 probability of giving a red ball

Urn X has a 3/7 probability of giving a black ball
Urn Y has a 4/9 probability of giving a black ball
Urn Z has a 1/2 probability of giving a black ball

P(2 red, 1 black) = P(Red Red Black) + P(Red Black Red) + P(Black Red Red)
                  = (4/7)(5/9)(1/2) + (4/7)(4/9)(1/2) + (3/7)(5/9)(1/2)
                  = 20/126 + 16/126 + 15/126
                  = 51/126
                  = 17/42
