A constable spots a pickpocket 250 m ahead of him and gives chase. The pickpocket runs at 8 km/h and the constable at 10 km/h. How many metres does the pickpocket cover before he is caught?

The gap closes at 10 − 8 = 2 km/h = 5/9 m/s, so the 250 m gap takes 250 ÷ (5/9) = 450 seconds to wipe out. In those 450 seconds the pickpocket, at 8 km/h = 20/9 m/s, covers 1000 m. (The constable covers 1250 m — exactly 250 m more, as he must — so quoting 1250 answers the question about the wrong runner.)