converging mirror = concave mirror.
real image = inverted image.
image size h' = - 4 cm
h = 1 cm
=> magnification m = h'/h = -4
as m = -v/u = -4 and u = -20 cm
=> v = 4 u = - 80 cm
write mirror equation and solve for focal length f.
1/f = 1/v + 1/u = -1/20 -1/80 = -1/16
f = -16 cm
