1. The problem statement, all variables and given/known data In the photo (which I cannot post now, but it is a magnifying glass, and a dollar bill) the images through the lens appears twice the size and it is at a distance of 3.0cm from the lens of the magnifying glass. Determine focal length. 2. Relevant equations 1. -di/do=m 2. 1/f=1/di+1/do 3. The attempt at a solution Since twice the size m will be 2. Using equation 1: di=-m*do = -2*3.00cm = -6.00cm This makes sense since image is upright and virtual, and in lenses virtual images are negative. When I calculate focal length, 1/f=1/-6+1/3 f=6 But this answer is wrong the answer is 3. What am I doing wrong or what do I not understand? Thanks!