Calculate the focal length of a planoconvex lens?

[givemeknowledge]

Homework Statement

Calculate the focal length of a planoconvex lens that produces a real image 2.5 times as big as the object if the object is 2.5 cm from the lens?

Homework Equations

The only one I can think of is 1/f = 1/v + 1/u

The Attempt at a Solution

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My 'attempt' is 1/f = 1/62.5 + 1/25 = 0.056 but this just seems totally wrong.

Had a whirl at it, but cannot produce an answer, any help would be really appreciated!

 

[Drakkith]

The only one I can think of is 1/f = 1/v + 1/u

I'm not familiar with u and v in these equations. I think my class labels them differently. What do they represent here?

 

[andrevdh]

You have to invert the answer to get f :)