A concave mirror having focal length of magnitude 20 cm forms a real image at a distance of 60 cm from it. The object distance (in cm) is:

Correct option is C

Given:

f = 20 cm (negative because it's a concave mirror so, -20)

v = 60 cm 

Formula Used:

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

Solution:

Substituting values into the mirror formula, we get

​$$\frac{1}{-20}=\frac{1}{-60}+\frac{1}{u}$$

Solve for object distance 

​$$\frac{1}{u}=\frac{1}{-20}+\frac{1}{60}$$

$$\frac{1}{u}=\frac{(-3+1)}{60}$$

$$\frac{1}{u}=\frac{-2}{60}$$​​​​

u = -30 cm