At what distance should an object be placed from a convex lens of focal length 15 cm to obtain a real image three times the size of the object?

20 cm

The correct answer is Option (2) → 20 cm

Magnification formula, $m=\frac{v}{u}=-3$ [Given]

$⇒v=-3u$

using lens formula,

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

$\frac{1}{15}=\frac{1}{-3u}-\frac{1}{u}$

$⇒u=-\frac{4×15}{3}=-20cm$