Concept: The question tests on the basics of Rates(Time and Work)
When moving in the same direction, the net effect of relative speed is the difference of speeds of the 2 objects.
In 2 hours, the thief has traveled 40 * 2 = 80 km
After 1 hour of chase, the police have gained 50 - 40 = 10km over the thief. At this point, the thief is 70km away.
During the 1/2 hour repair, the thief gets 40 * 1/2 = 20 km over the police. So he is now 90 km away from the police.
Let the police jeeps speed be x. T = Distance / Relative Speed
Therefore 4.5 = 90 / (x - 40)
4.5x - 180 = 90
4.5x = 270
x = 2700 / 45 = 60 km/hr