Question

at what distance shuold an object be placed in front of convex lens of focal lenght 20 cm to obtain an image of double its size?

Answer 1

Answer on Question 38198, Physics, Options Formula for magnification is

$M=\frac{f}{f-d_{o}}$

where $f$ is focal length and $d_{o}$ is the distance from the lens to the object. So we can find

$d_{o}=f-\frac{f}{M}=20-20/2=10\,cm$

The answer is 10 cm.