Given that,
Radius of mirror = 20 cm
Size of image = 3 times of size of object
We need to calculate the distance of the screen from the mirror
Using formula of magnification


Where, h' = size of image
h = size of object
v = image distance from mirror
u = object distance from mirror
Put the value into the formula



The distance of image will three times of object distance.
Hence, The distance of the screen from the mirror is 3u.