math
posted by Lindsay .
A motorist travelling at a steady speed of 120km/h covers a section of the highway in 30min. After a speed limit is imposed he finds that when travelling at the maximum speed allowed he takes 20 min. longer than before to cover the same section.
Calculate the speed limit.

If he travels at 120 Km per hour in 30'(ie 1/2 hour) he did 120/2=60Km.
If he travels at max speed allowed he takes 30+20=50'or 50/60=5/6 hours to reach his destination. so speed is: 60/(5/6)= 360/5=72Km/h