To determine the focal length of a lens, you place the lens in front of a small lightbulb and then adjust a viewing screen to get a sharply focused image. Varying the lens position produces the following data:

Bulb to lens (cm) ..................Lens to screen (cm)
20.......................................................61
22.......................................................47
24.......................................................39
26.......................................................37
28.......................................................32
Use the best-fit line of an appropriate graph to determine the focal length of the lens.