f= Focal length of concave mirror =20cm
do = Distance of object from concave mirror = 40cm
di = Distance of image from concave mirror = ?
According to concave mirror formula
1/f = 1/do + 1/di
So, 1/di = 1/f - 1/do
= 1/20 - 1/40 = 1/40
Thus, di = 40cm in front of the concave mirror.
But , a plane mirror is palced at a distance of 20cm, so image rays after reflection from plane mirror goes to concave mirror and formed image at Infinity. Since Image formed of object placed at focus is infinity.
The final position of the image is infinity