A concave mirror of focal length f produces a real image n times the size of the object. The distance of the object from the mirror is

$\text{n}=\frac{\text{size}\text{of}\text{the}\text{image}}{\text{size}\text{of}\text{the}\text{object}}$ 

In case of a concave mirror, if the image is real, then it must be inverted.

$\text{m}=-\text{n}=-\frac{\text{v}}{\text{u}}$

$\text{m}=\text{n}=\frac{\text{v}}{\text{u}}$

From mirror formula

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

$1+\frac{\text{1}}{\text{n}}=\frac{\text{u}}{\text{f}}$

$\text{u}=\left(\frac{\text{n}+1}{\text{n}}\right)\text{f}$