The distance covered by the policeman in 12 sec

s = ut + 1/2 at2

= 0 x12 + 0.5 x3 x 12x12 = 216m

v = u + at = 0 + 12x3 = 36m/s

The distance covered by the robber in 12 sec

s = 20 x 12 = 240m

Suppose he over takes in 12 + t sec, in which he travels = 216 + 36 * t

r robbers will travel in the same time 20 * ( 12+t) m

216 + 36 t = 240 + 20 t

t = 24 / 16 = 1.5 s

Distance travelled by policeman in 12 + 1.5 = 13.5 s

= 216 + 36 x 1.5 = 270 m