The sine rule will help you here.
On a unit circle, sin(Θ) would be the y-coordinate of point P (ie., 20). But we're not on the unit circle, so we'll have to divide 20 by the length of P (from the origin). The length of P = √(15²+20²) = 25.
So sin(Θ) = 20/25 = 0.8
