Points A, B, and C lie, in that order, on a straight railroad track. The distance from point A to point B is twice the distance from point B to point C. A train traveled from point A to point C without stopping. The train's average speed when traveling from point A to point B was 50 miles per hour and the train's average speed when traveling from point B to point C was 25 miles per hour. What was the train's average speed, in miles per hour, when traveling from point A to point C? Options
A) 20 B) 25 C) 30 D) 35 E) 15
Average speed = distance/time   Because we are looking for average speed we can pick a distance for the variable d.   Speed A-B = 50   Speed B-C = 25   Average Speed = total distance/total rate   rate = distance/time   A====================B==========C   If A-B is twice the length of B-C then let A-B = 2d and let B-C = d   Average speed = 3d/(2d/50) + (d/25)   3d/(2d/50) + (2d/50)   3d/(4d/50)   150d/10d   Average speed = 15   ANSW15R: 15150 /10= 
<gadget id="calculator">150 /10</gadget>
<output>15</output>
15   ANSW15R: 15
<result>E</result>
Points A, B, and C lie, in that order, on a straight railroad track. The distance from point A to point B is twice the distance from point B to point C. A train traveled from point A to point C without stopping. The train's average speed when traveling from point A to point B was 50 miles per hour and the train's average speed when traveling from point B to point C was 25 miles per hour. What was the train's average speed, in miles per hour, when traveling from point A to point C?