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).231
C).538
D).462

Question

Quizplus · Accepted Answer

The probability that one of the two phones is faulty can be calculated by considering the total ways to choose one faulty phone and one working phone out of the total, divided by the ways to choose any two phones out of the total. There are 4 faulty phones and 9 working phones. The number of ways to choose one faulty and one working phone is 4*9 = 36. The total number of ways to choose any two phones out of 13 is 13C2 = (13*12)/(2*1) = 78. Therefore, the probability is 36/78 = 0.461538, which rounds to 0.462.

As Part of a Promotion, Both You and Your Roommate