The real image of an object is obtained from a mirror of focal length 20 cm which gives magnification 3. Find object distance.

Dear Student ,
Here in this case from the equation of spherical mirror we can write that ,
$$\begin{gathered} \frac{1}{v}+\frac{1}{u}=\frac{1}{f} \\ \Rightarrow \frac{1}{u}\left(1+\frac{u}{v}\right)=\frac{1}{f} \\ \Rightarrow \frac{1}{u}\left(1+\frac{1}{3}\right)=\frac{1}{20} \left[\because \frac{v}{u}=3 so, \frac{u}{v}=\frac{1}{3}\right] \\ \Rightarrow \frac{4}{3u}=\frac{1}{20} \\ \Rightarrow u=\frac{80}{3}=26·66 cm \end{gathered}$$
So here the object distance is 26.66 cm .
Regards