Question

There is one error in one of five blocks of a program. To find the error, we test three randomly selected blocks. Let X be the number of errors in these three blocks. Compute E(X) and Var(X).

Answer 1

The probability, or expectation value for X in a block, equals the number of occurrences of X in all the blocks divided by the total number of blocks

well E(X)=µ and 

Var(X)=E[(X-µ)^2]