Converging Lens

[dmolson]

1. The problem statement, all variables and given/known data

The distance between an object and its image is fixed at 40.0 cm. A converging lens of focal length f = 4.2 cm forms a sharp image for two positions of the lens. What is the distance a between these two positions?

2. Relevant equations

1/p + 1/q = 1/f

3. The attempt at a solution

I am very confused with this problem and really have no idea where to begin. If anyone can provide some assistance, that would be great. Thanks.

[hage567]

You have the lens equation. You know f. You also know that p+q=40. So what you have is two equations and two unknowns. Try solving for p (or q) and see what you come up with.