7.20 For a sheet-metal stamping machine in a certain factory, the time between failures, X1, has a mean (MTBF) of 56 hours and a variance of 16.

The repair time, X2, has a mean (MTTR) of 5 hours and a variance of 4.

a If X1 and X2 are independent, find the expected value and variance of Y  X1  X2, which represents one operation/repair cycle.

b Would you expect an operation/repair cycle to last more than 75 hours? Why or why not?