In this question let's assume that the thief is caught after time T after being spotted by the policeman
Now in this time 'T' let's suppose the thief covers X meters. Since the policeman was 200 meters behind the thief, he needs to cover 200+X meters in this time 'T'. Therefore we get the following equation:
X/5=200+X/7
on solving this equation we get X=500 meters