A convex lens produces a double size real image when an object is placed at a distance of 18 cm from it. Where should the object be placed to produce a triple size real image ?

Explanation

`u=-18cm,m=-2`
`m=(v)/(u)implies-2=(v)/(-18)`
`v=36cm`
`u'=-x,m^'=-3`
`m'=(v')/(u')rArr-3=(v')/(-x)`
`v'=3x`
`(1)/(f)=(1)/(v)-(1)/(u)=(1)/(v')(1)/(u')`
`(1)/(36)-(1)/(-18)=(1)/(3x)-(1)/(-x)`
`(3)/(36)=(4)/(3x)`
`x=16cm`