# Distance of object/lens

The homework problem is "A sharp image is located 68.0 mm behind a 56.0 mm focal length converging lens. Calculate the object distance in millimeters"

1/f = 1/di + 1/do

I tried solving based on having focal (f) and distance of image (di) so 1/56 mm = 1/(68 mm + 56 mm) + 1/do
I solved for do coming up with 102.04 mm and the computer doesn't accept

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