How Do You Calculate Object Distance in Lens Equations?

[sp1974]

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

 

[Squeezebox]

Why is d_i=68+56?

 

[sp1974]

I figured because the focal point was 56 and it says 68 behind it so I thought it would be f distance + the 68

 

[Squeezebox]

It states it is behind a 56mm focal length lens. Not 68mm behind the focal length of a 56mm focal length lens.

 

[sp1974]

I also did it as 1/56 = 1/68 + 1/do and the computer didnt accept that on as well

 

[sp1974]

ok I figured this one out. I had my numbers flip flopped my numbers when subtracting