Robert walks from point A to point B at an average speed of 10 kilometers per hour. At what speed, in kilometers per hour, must Robert walk from point B to point A so that her average speed for the entire trip is 12 kilometers per hour?	Choose one:
A) 15km/hr B) 21km/hr C) 16km/hr D) 20km/hr E) 17km/hr
Let's suppose that speed while returning was x km/h   Since the distance is same, we can apply the formula of avg speed   15km/hrvg speed= 2S1S2/S1+S2   12= 2*10*x/10+x   x= 15 km/hr   answer is 15km/hr
<result>A</result>
Robert walks from point A to point B at an average speed of 10 kilometers per hour. At what speed, in kilometers per hour, must Robert walk from point B to point A so that her average speed for the entire trip is 12 kilometers per hour?