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

Explanation

Here, `u = ?, f = 15 cm, m = +- 3 = (v)/(u)`
:. `v = +- 3 u`
From `(1)/(v) - (1)/(u) = (1)/(f)`
`(1)/(+- 3 u) - (1)/(u) = (1)/(15)`
:. `u = 20 cm`, when `m` is negative,
`u = 10 cm`, when `m` is positive,
i.e., for virtual image, `u = 10 cm` and for real image, `u = 20 cm`.