An object is placed at a distance of 40 cm from a convex lens of focal length 30 cm. Find the position of the image.

We know that,  

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

=> $\frac{1}{v} = \frac{1}{f} + \frac{1}{u} = \frac{1}{30} + \frac{1}{-40} = \frac{1}{30} - \frac{1}{40}$

=> $v = \frac{1200 }{ 10}$

= 120cm.