What is the Focal Length of a Convex Lens for Sharp Image Formation?

[wakejosh]

Homework Statement

An object and a screen are separated by 20 cm. A convex lens is placed between them, 5 cm from the object. In this position it causes a sharp image of the object to form on the screen. What is the focal length of the lens?

The Attempt at a Solution

wouldn't it just be 5 cm since it is producing a sharp image on the screen? the distance from the object to the lense?

 

[wakejosh]

looking at this more I think i need an equation. I don't have a book so can someone show me the equation I am looking for?

 

[hage567]

You need the thin lens equation:

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

where f is the focal length, p is the object distance, and q is the image distance.