Answer :

Correct answer is : 15

m = $-<!--\; -\; -->\frac{v}{u}$ = $+\frac{1}{2}$

v = $-<!--\; -\; -->\frac{u}{2}$

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

$\frac{2}{-<!--\; -\; -->u}+\frac{1}{u}=\frac{1}{f}$

u = – f = –15 cm