Question

If 4 bulbs are there each of which can be defective or non-defective then what is the probability that at least one bulb is non- defective?

(a) 7/8

(b) 15/16

(c) 3/4

(d) 1/2

This question was addressed to me at a job interview.

This intriguing question originated from Probability in chapter Probability of Mathematics – Class 11

Answer 1

Right answer is (b) 15/16

To explain I would say: Each bulb can have two cases i.e. either defective or non-defective. Then total number of outcomes will be 2^4 = 16.

Probability that all the bulbs are defective is 1/16.

So, the probability that at least one bulb is non-defective is 1-1/16 = 15/16.