The correct option is (A) 30 cm.
Explanation:
image size = 3 × object size
∴ m = (v/u) = 3
given: u + v = 80cm
∴ u + 3u = 80cm
∴ u = 20cm
∴ v = 60cm as image is erect, image must be virtual
∴ u = – 20cm
v = 60 cm
using (1/v) + (1/u) = (1/f)
∴ (1/f) = (1 / 60) – (1 / 20)
∴ (1/f) = (– 2 / 60) = (– 1 / 30)
∴ f = – 30cm
∴ Concave mirror with f = 30cm.
