Question

The size of the image formed by a convex mirror of focal length 30 cm is a quarter of the size of a real object. Then the distance of the object from the mirror is

a
30 cm

b
90 cm

c
120 cm

d
60 cm

Answer 1

size means magnification

mirror formula

(1/v) + (1/u) = (1/f)

Make sure you don't make any sign mistake

magnification = -v/u = 1/4

v-->image distance
u -->object distance
f-->focal length

Now you know mirror formula and you know magnification. Your job is to write v in terms of f and u and subsitute in magnification formula. Then finally find u

Answer 2

LOL. I forgot my usual warning --> MAKE SURE YOU WRITE ALL QUANTITIES IN SI UNITS before substituting