If you want to produce an image of a bulb that is enlarged by a factor of 2, how far from the wall should the lens be placed. Focal length is 40cm. I know you use 1/d_o + 1/d_i =1/f and m= -d_i/d_o but how do you find d_o? -2/d_o +1/d_o =1/f ? but it comes out to 40cm and not 60cm for d_o. If someone could help me with that bit I can solve it from there. Thanks!!!