An object is placed $30$ cm from a converging lens of focal length $20$ cm. Find the magnification of the image.

$\dfrac{1}{v} = \dfrac{1}{20} - \dfrac{1}{30} = \dfrac{1}{60}$, so $v = 60$ cm. Magnification $m = \dfrac{v}{u} = \dfrac{60}{30} = 2$; the image is real, inverted and twice the object's size. Using $\dfrac{f}{u}$ gives 0.67 and $\dfrac{v}{f}$ gives 3.