Brent@GMATPrepNow wrote: ↑

Sun Aug 28, 2022 12:25 pm

There are 3 boxes labeled A, B and C. Box A contains 6 coins, 1 of which is made of gold. Box B contains 4 coins, 1 of which is made of gold. Box C contains 2 coins, 1 of which is made of gold. If a box is randomly selected, and a coin is randomly selected from that box, what is the probability the selected coin is made of gold?

A) 1/12
B) 1/4
C) 11/36
D) 1/3
E) 5/12

Answer: C
Source: www.gmatprepnow.com

P(selected coin is made of gold) = P(select box A AND select gold coin from box A OR select box B AND select gold coin from box B OR P(select box C AND select gold coin from box C)

= [P(select box A) x P(select gold coin from box A)] + [P(select box B) x P(select gold coin from box B)] + [P(select box C) x P(select gold coin from box C)]

= [1/3 x 1/6] + [1/3 x 1/4] + [1/3 x 1/2]

= 1/3(1/6 + 1/4 + 1/2)

= 11/36

Answer: C