A bus completed first 50 miles of a 120-mile trip at an average speed of 20 mph. Then it took a halt of 30 minutes and completed the half of the remaining journey at an average speed of 35 mph. At what average speed it should complete the remaining journey so that the overall average speed of the whole journey becomes 20 mph?

Wanted overall average speed of journey = 20 mph

Total distance = 120 miles

Time required to cover for total distance = 120/20 = 6 Hours

First 50 mile

20 mph => Time taken for trip = 50/20 = 2.5 Hours

Halt of 30 minutes or 0.5 Hour.

Remaining distance = 120 - 50 = 70 miles.

Half of remaining journey = 70/2 = 35 miles at 35 mph => Time taken for trip = 35/35 = 1 Hour

Time Elapsed so far = 2.5 + 0.5 + 1 = 4 Hours

Time that should be spent to achieve 20 mph = Time required to cover for total distance - Time Elapsed so far => 6 - 4 = 2

Remaining distance 70 - 35 = 35 miles

Average Speed = 35/2 = 17.5 mph

Answer E