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?	Select:	A) 55 km/h	B) 65 km/h	C) 85 km/h	D) 75 km/h	E) 90 km/h
Let d be the distance bewteen 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 is 75 km/h)75 km/h
<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?