What Focal Length Lens Corrects a Near Point from 80cm to 30cm?

[klilly]

Homework Statement

A person had a near point of 80 cm. What focal length lens would be required to correct this persons near point to 30cm, allowing then to read at this distance

Homework Equations

Maybe: 1/di + 1/do = 1/f = P
I found another similar question which used 1/(the given near point aka the 80cm) - 1/(the desired near point aka 30) = 1/f
This got me the right answer, but I still don't understand why.

The Attempt at a Solution

The answer is 48cm

 

[member 553083]

. I got this answer by using the equation I found for similar problems, which is 1/(the given near point aka the 80cm) - 1/(the desired near point aka 30) = 1/f