Okay, so I have this question that has been killing me. This is it:

Iffis the focal length of a convex lens and an object is placed at a distancepfrom the lens, then its image will be at a distanceqfrom the lens, wheref,p, andqare related by the lens equation

1/f=1/p+1/q

What is the rate of change ofpwith respect toqifq=4 andf=1 ?

So I've tried about a million approaches and I never get the right answer. I know you're supposed to show how you've attempted this problem, but I've tried so many ways I don't know which one to type on here. Basically, I'm hoping somebody will be able to give me a boot in the right direction because I actually haven't a clue as to how to even begin attacking this.

Any hints?

Thanks!