Lensmaker's Formula: Solving for Image Distance i

[jpartdq]

Homework Statement

A lens has a focal length of f=20cm. If an object is at o=50 cm, where is the image?

Homework Equations

1/f = 1/o + 1/i

The Attempt at a Solution

1/20 = 1/50 + 1/i

5/100 = 2/100 + 1/i

3/100 = 1/i

i = 100/3

I'm not sure how the answer could be 1/20 = 1/50 + 1/(100/3)? What am I doing wrong?

 

[ideasrule]

The answer is i=100/3 cm (i is the image distance). Why do you suspect it's wrong?