So the whole problem is:

If $\displaystyle f$ is the focal length of a convex lens and an object is placed at a distance $\displaystyle q$ from the lens, then its image will be at a distance $\displaystyle p$ from the lens, where $\displaystyle f,g,$ and

$\displaystyle p$ are related by the lens equation

$\displaystyle \frac{1}{f}=\frac{1}{q}+\frac{1}{p}$

Suppose the focal length of a particular lens is 20cm. What is the rate of change of $\displaystyle q$ with respect to $\displaystyle p$ when $\displaystyle p=3$?

So to start out I plug in $\displaystyle f=20cm$ and $\displaystyle p=3$ 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!