A policeman follows a thief, who is 150 metres

Distance between policeman and thief = 150 m
Speed of policeman = 6 km/h = $ \frac{6000}{3600} = \frac{5}{3} $ m/s
Speed of thief = 5 km/h = $ \frac{5000}{3600} = \frac{25}{18} $ m/s
Relative speed = $ \frac{5}{3} - \frac{25}{18} = \frac{5}{18} $ m/s
Time to catch up = $ \frac{150}{5/18} = 150 \times \frac{18}{5} = 540 $ s
Distance covered by policeman = $ \frac{5}{3} \times 540 $= 900 m
Answer: 900 m