1. The problem statement, all variables and given/known data An object is located to the left of a thin lens. The image is upright, and the distance between the object and image is 20 cm. What are the focal length and object distance if the lens is 2 times the hieght of the object? 2. Relevant equations 1/p + 1/q = 1/f 3. The attempt at a solution I actually know how to get the focal length and object distance, I'm just having a bit of difficulty with an initial step. If the image and object are 20 cm apart. How do I determine what the relationship between them is? I know the image location (q) is negative and the object (p) is positive. According to my book, I should use 20 cm = -q - p. That isn't immediately obvious to me, I would have thought doing something like 20 = p - q or 20 = p + q... can anyone explain why it's -q - p for the distance differential?