1. The problem statement, all variables and given/known data Calculate the focal length of a planoconvex lens that produces a real image 2.5 times as big as the object if the object is 2.5 cm from the lens? 2. Relevant equations The only one I can think of is 1/f = 1/v + 1/u 3. The attempt at a solution My 'attempt' is 1/f = 1/62.5 + 1/25 = 0.056 but this just seems totally wrong. Had a whirl at it, but cannot produce an answer, any help would be really appreciated!