One of the practice problems in my book is giving me trouble.

It asks

"A hemispherical bowl of radius 10 cm contains water to a depth of h cm. Find the radius of the surface of the water as a function of h."

I can see that the radius gets smaller as h gets smaller but I'm not how to go about finding an equation. The back of the book says that the answer is r=(20h -h^2)^(1/2) and I think they used the circle equation but I'm not sure. Does anyone have any hints?