A motor car starts with the speed of 70 km/hr with its speed increasing every two hours by 10 kmph. In how many hours will it cover 345 kms?

A motor car starts with the speed of 70 km/hr with its speed increasing every two hours by 10 kmph. In how many hours will it cover 345 kms?
[A] 2 (1/4) hrs
[E] None of these
[C] 4(1/2) hrs
[B] 4 hrs 5 min
[D] Can not be determined
ANS : C
Explanation:
Distance covered in first 2 hours = (70 x 2) km = 140 km
Distance covered in next 2 hours = (80 x 2) km = 160 km
Remaining distance = 345 – (140 + 160) = 45 km.
Speed in the fifth hour = 90 km/hr
Time taken to cover 45 km =(45/90) hr = (1/2) hr
Therefore Total time taken = 2 + 2 + (1/2) = 4 (1/2)hrs