A car travels first 160 km at 75 km/hr and the next 160 km at 70 km/hr. What is the average speed for the first 320 km of the tour?  Select:
A) 71.11  B) 71.12  C) 72.4  D) 71.17  E) 71.13

Rationale: "car travels first 160 km at 75 km / hr time taken to travel first 160 km = distancespeed = 160 / 75 car travels next 160 km at 70 km / hr time taken to travel next 160 km = distancespeed = 160 / 70 total distance traveled = 160 + 160 = 2 × 160 total time taken = 160 / 75 + 160 / 70 average speed = total distance traveled / total time taken = 320 / ( 160 / 75 + 160 / 70 ) = 72.4 km / hr answer : c"
Answer: C