Possible values of X, the number of components in a system submitted for repair that must be

Question:

Possible values of X, the number of components in a system submitted for repair that must be replaced, are 1, 2, 3, and 4 with corresponding probabilities .15, .35, .35, and .15, respectively.
a. Calculate E(X) and then E(5 - X).
b. Would the repair facility be better off charging a flat fee of $75 or else the amount $[150/(5 - X)]?