Minimizing Variance of a Linear Combination

Let X and Y be independent random variables with $E(X) = 12$, $E(Y) = 8$, $\text{Var}(X) = 16$, and $\text{Var}(Y) = 25$. A new random variable is defined by the linear combination $Z = aX + bY$, where $a$ and $b$ are constants. If $E(Z) = 20$ and the variance of Z is minimized, what are the values of $a$ and $b$?