A motor cyclist participant of a race says "We drove with the speed of 15 miles an hour one way, but while returning because of less traffic we drove on the same route with 21 miles per hour." What was their average speed in the whole journey? Options:
A) 10 miles/hour B) 12 miles/hour C) 11 miles/hour D) 13 miles/hour E) 17.5 miles/hour
2xy/x+y=17.5 miles/hour   ANSW17.5 miles/hourR:17.5 miles/hour
<result>E</result>
A motor cyclist participant of a race says "We drove with the speed of 15 miles an hour one way, but while returning because of less traffic we drove on the same route with 21 miles per hour." What was their average speed in the whole journey?