So the whole problem is:

If is the focal length of a convex lens and an object is placed at a distance from the lens, then its image will be at a distance from the lens, where and

are related by the lens equation

Suppose the focal length of a particular lens is 20cm. What is the rate of change of

with respect to

when

?

So to start out I plug in

and

into the equation above but then I can't figure what to do from here. I just having trouble comprehending the problem and figuring out how to start. Any help is greatly appreciated!