I'm having a little problem with this question.. my brain is too small. "A converging lens has a focal length of 10 cm. A screen is placed 30 cm from an object. Where should the lens be placed, in relation to the object, to produce a focused image?" I've started with the formula 1/do + 1/di = 1/F 1/do + 1/di = 1/10 At this point I don't have a distance for do or di, because they're relative to the lens, and the lens position is the variable. Can someone push me in the right direction?