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

E for expectation?

Answer 2

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

Answer 3

so that would be 1/5, but we get 3 selected blocks, so this would mean 1/5 * 3 = 0.6 (which would be what my book sais).

If this is correct, how do i calculate the Var(X) from this?

Answer 4

well E(X)=µ

and
Var(X)=E[(X-µ)^2]

Answer 5

Var(X) = E[(X-µ)^2]
= E[X^2]-µ^2