The intercepted arc is twice the size of the inscribed angle. For instance, here we have the inscribed angle with 50° so x(intercepted arc) = 2*50 = 100°
The other example is the intercepted arc given and inscribed angle needed. So divide 40° by 2 we will get 20°. So y = 20°
x = 100°
y = 20°
Hope it helps :)