Question

when the object is at a distance of 40 cm from the concave mirror, the size of the image is equal to the size of the object.. The focal length of the mirror is.....

Answer 1

There is only one case when object size is same as image size , that case is , when object is at center of curvature , so here center of curvature is at 40cm away from concave mirror

So the radius is 40 cm

we know focal length is half of radius = 1*40/2 = 20cm

Answer 2

Hope this will help you