Hi Mary, the idea is that a yacht is essentially a deformed sphere. Spheres have surface area 4 * pi * r^2.
So we can do
50^2 = 40^2 + 30^2
and multiply both sides by 4 * pi to get
4*pi*50^2 = 4*pi*40^2 + 4*pi*30^2
which says "A ball of radius 50 has the same surface area as a ball of radius 40 and a ball of radius 30". In our case, we're using ship hulls, vs. perfect spheres, but the relationship is (theoretically) the same.