Problem 15. (2.5 %) Suppose X is uniformly distributed over [0,4] and Y is uniformly distributed over [0,1]. Assume X and Y are independent. Let Z = X + Y. Then
 (i) fz(4.5)=0
 (ii) fz(4.5)=1/8
 (iii) fz(4.5) =1/4
 (iv) fz(4.5)=1/2