1. The problem statement, all variables and given/known data When an object is placed 69.0cm from a certain converging lens, it forms a real image. When the object is moved to 39.0cm from the lens, the image moves 19.0cm farther from the lens. Find the focal length of this lens. 2. Relevant equations 1/do + 1/di = 1/f 3. The attempt at a solution 1/69.0cm + 1/di = 1/f 1/39.0cm + 1/di + 19cm = 1/f 1/69.0cm + 1/di = 1/39.0cm + 1/di + 19cm which gives di = 20cm 1/69.0 cm + 1/20.0 cm = 1/f f = 15.5??? this is not the right answer though, where am i going wrong?