A convex mirror of radius of curvature 30 cm forms an image that is half the size of the object. The object distance is :

Given R = 30 cm

f = R/2 = +15 cm

Magnification $\left(\mathrm{m}\right)=\pm \frac{1}{2}$

For convex mirror, virtual image is formed for real 
object. 
Therefore, m is +ve 

$\frac{1}{2}=\frac{f}{f-u}$

u = – 15 cm