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How big will the image of that person be on film?

How far behind the lens (assumed to be very thin) must the film be placed?

I tried using the thin-lens equation and the equation of magnification, but I can't seem to be getting the right answer.

(1/d o)+(1/d i)=(1/f)

m=-(d i)/(d o)

Using the first equation, I plugged in the numbers for f and d o and solved for d i. I know that there is something special with the signs, but I tried them every way possible, and I still can't get the right answer.