light refraction.

given: h = 4 cm; h' = - 6 cm; u + v = 20 cm; f = ?

since m = h' / h = v / u

- 6 / 4 = v / u

v = - (3 / 2) u

So u - (3 / 2)u = 20

- (1 / 2) u = 20

u = - 40 cm; and v = 60 cm  

Using lens formula

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

(1 / f) = (1 / 60) + (1 / 40)

(1 / f) = (2 + 3) / 120

f = 120 / 5 = 24 cm

lens is convex.