[Exer15-2] Exercise 28 of Book {Devore2017B} P.221
Show that if $X$ and $Y$ are independent rv's, then $E(XY)=E(X)\cdot E(Y)$. Then apply this in Exercise 25(Here is Question2504). [{\it Hint:} Consider the continuous case with $f(x,y)=f_X(x)\cdot f_Y(y)$.]