A concave mirror is a converging mirror.
Distance of the object from the mirror 'u' = -20 cm
Height of the object 'ho' = 1 cm
Height of the image 'hi' = -4 cm
We have to find the distance of the image 'v' and focal length of the mirror 'f'.
Using the magnification formula, we get
Thus, the distance of the image 'v' is 80 cm.
Now, using the mirror formula, we get
Thus, the focal length of the concave mirror is 16 cm.