$P(50pt)=\frac{\text{area of success}}{\text{total area}}=\frac{\pi(1)^{2}}{\pi(4)^{2}}$

$\frac{1}{16}$

$P(20pt)=\frac{\text{area of success}}{\text{total area}}=\frac{\pi(2)^{2}-\pi(1)^{2}}{\pi(4)^{2}}$