Originally Posted by

**mr fantastic** I made a mistake earlier. It's not geometric.

The probability that you need 1 test (that is, you get the faulty component in the first selection) is 1/5.

The probability that you need 2 tests (that is, you get the faulty component in the second selection) is (4/5)(1/4) = 1/5.

The probability that you need 3 tests (that is, you get the faulty component in the third selection) is (4/5)(3/4)(1/3) = 1/5.

etc.

So E(number of tests) = 1(1/5) + 2(1/5) + 3(1/5) + .... + 5(1/5) = 15/5 = 3.

Get the variance in a similar way.