A car travels from A to B at an average speed of 50 km/hour. At what average speed would it have to travel from B to A to average 60 km/hour for the whole trip?	Pick one
A) 65 km/hour  B) 95 km/hour  C) 55 km/hour  D) 75 km/hour  E) 45 km/hour
Let d be the distance between A and B   T1 = d / 50 : travel time from A to B   Let S be the speed from B to A   T2 = d/S : travel time from B to A   60 = 2d/(T1 + T2) : average speed for the whole trip   60 = 2d/(d/50 + d/S) : substitute T1 and T2   S = 75 km/hour : solve the above equation for S.   correct answer 75 km/hour
<result>D</result>
A car travels from A to B at an average speed of 50 km/hour. At what average speed would it have to travel from B to A to average 60 km/hour for the whole trip?