As part of a promotion, both you and your roommate are given free cellular phones from a batch of 13 phones. Unknown to you, four of the phones are faulty and do not work. Find the probability that one of the two phones is faulty.
A) .077
B) .538
C) .231
D) .462

Question

Quizplus · Accepted Answer

The probability that one phone is faulty and the other is not can be calculated using combinations. There are C(4,1) ways to choose 1 faulty phone and C(9,1) ways to choose 1 non-faulty phone. The total ways to choose 2 phones from 13 is C(13,2). The probability is [C(4,1) * C(9,1)] / C(13,2) = (4 * 9) / 78 = 36/78 = 0.462.

As Part of a Promotion, Both You and Your Roommate