Any help as to how to get started or get through the problem will be much appreciated:

An oil spill is spreading in a circular pattern from a damaged tanker on a calm sea. The radius of the circle is growing at a rate of r'(t)=40 times the radical of t feet per minute.

A)How much does the radius grow from time t= 100 to t= 225?

B) When will the radius be 1,000 feet long?