Help Solve Lens Projector Distance Puzzle!

[cuddlylover]

I have this in my home work and i am not 100% on if i have it right or not

A converging lens with a focal length of 25 cm is used to make a slide projector. If the image is to be projected onto a screen 10m away, how far should the lens be located from the transparency?

I did it like this:

1/s + 1/1000cm = 1/25cm

solve for s

1/s = 1/25cm - 1/1000cm
∴ s = 25.64cm (2dp)

Is this right or am i way off. If some one could help would be a big help thanks.

 

[cuddlylover]

Or should it be 3.9mm i am a bit lost here i think lol.

 

[Doc Al]

Looks OK to me.

 

[Doc Al]

Or should it be 3.9mm i am a bit lost here i think lol.

Where did you get that value?

 

[cuddlylover]

I posted this somewhere else and someone posted that but could not see how they got to it :)