1. The problem statement, all variables and given/known data When an object is placed at the proper distance to the left of a converging lens, the image is focused on a screen 30.0 cm to the right of the lens. A diverging lens is now placed 15.0 cm to the right of the converging lens, and it is found that the screen must be moved 19.2 cm farther to the right to obtain a sharp image. What is the focal length of the diverging lens? 2. Relevant equations 3. The attempt at a solution (sorry my poor english) I dont know how to solve this problem using calculus. Then I've used the trick shown below and I've found approximately the correct answer (the error was caused by an incorrect alignment in the sketch). My doubt is if it's correct to use this method for finding the solution. Since I got the right answer, I think it is valid. But also, I was unable to draw the image in the second situation, it would be at the location of the blue dot.... but the rays does not intercept each other.