(1)       

             Given R = 30 cm

              $\Rightarrow f=\frac{R}{2}=+15 cm$

             Magnification (m) = $\pm 1/2$

             For convex mirror, virtual image is formed for real object, therefore, $m$ is +ve

             The mirror formula,

              $\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\Rightarrow v=\frac{uf}{u-f}$

              Magnification, $m=-\frac{v}{u}=-\frac{uf}{u(u-f)}=\frac{f}{f-u}$

             $\frac{1}{2}=\frac{f}{f-u}=\frac{15}{15-u}\Rightarrow u=-15 cm$

(3)

Magnification, $m=2=\frac{-v}{u}$

$2=\frac{-(15-u)}{-u}$

$2u=15-u$

$3u=15\Rightarrow u=5\text{ cm}$

$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}=\frac{1}{10}+\frac{1}{-5}=\frac{1-2}{10}=\frac{-1}{10}$

$f=-10\text{ cm}$