mathematical optimization – Maximization of piecewise function

I’m trying to maximize piecewise functions with parameters but haven’t been able to do so. Here is a minimum example:

f(x_, a_) = -(x - 10*a)^2 + 50*a;
w(x_, a_) = (Piecewise) {
    {f(x, a), 1/2 < a < 8/x && 0 < x < 20},
    {-10, True}

I want to maximize $w$ over $x$ ($a$ is a parameter). Any ideas on how to solve this symbolically (or even numerically with $a$ in a grid)?