I came across this problem in my physics class, but I don´t how to solve it. The problem is:
A magnifying glass is used to produce a 4x magnification image. What is the focal distance of its lens?
I know the answer is $+6,25\,$cm, but I don´t know how to solve it.
I´ve tried using the lens formula $\,1/f = 1/d + 1/d_0\ $ and combining it with the magnification formula $M = -d_0/d$, but it did not work.