just saying

E[x]=450 minutes is OK for a month var(X)=4500 minutes is the number you get from calculaltions, but does it make any sense as a variance?

for 3: to me it looks that knowing Z1 doesn't inform you for Z2 and vice versa, so they look independent variables to me -> Cov(z1,z2)= 0. (couldn't figure anything else and made up this nice explanation to comfort my self