In 1 min distance covered by thief =100*1=100m
therfor gap at starting between thief and police=100m
when police start chasing his speed in 1st min is same as that of thief therfor ther is no chnge in the gap.
In the 2nd min of chase speed difference is 10 therfor the gap also reduces by 10 m
Similarly the gap reduces by 20 m in 3rd min.
thefor it takes police 5 min after he starts to catch the thief.
Therfor policeman takes total of 6 min after the thief starts running.