related rates - still need help!

A ruptured oil tanker causes a circular oil slick on the surface of the ocean. When its radius is 150 meters, the radius of the slick is expanding by 0.1 meter/minute and its thickness is 0.02 meter. At that moment:

(a) How fast is the area of the slick expanding?

(b) The circular slick has the same thickness everywhere, and the volume of oil spilled remains fixed. How fast is the thickness of the slick decreasing?

the answer to (a) is (i hope)

dA/dt= 2(pi)r (dA/dt)

dA/dt|r=150 = 30pi

so for (b).. the area of a cylinder is the area of the top times the height, but i dont know to go from there... do i find dh/dt or somethin? how do i do that?