Please help me solve this..

Suppose n balls are distributed at random into r boxes. Let Xi=1 if box i is empty and let Xi=0 otherwise.

a)Compute E(Xi) – answer: (1-1/r)^n

b)for i not equal j compute E(Xi,Xj) -answer: (1-2/r)^n

c) Let Sr denote the number of empty boxes. Write Sr=X1+X2...

and use result of a to compute E(Sr) -answer: r(1-1/r)^n

d) Use the result of a and b and compute Var(Sr)?????

I’m stuck on d… how do I compute var(Sr)???

thanks