Question

at what distance from concave
mirror of focal length 2.5 cm should a boy stand so that this image has a hight equal to the half of the original height

Answer 1

given: v = u/2
where v is the distance of image and u is the distance of object
f= 2.5 cm
applying mirror formula.
1/v+1/u=1/f
u for concave mirror u and v is negative
-2/u-1/u=1/f
-3/u=1/2.5
u=-7.5
so, it has to be placed 7.5 in front of the lens.