A boat round trip in a river goes upstream at an average speed of 30 miles per hour. On the trip back downstream boat goes at an average speed of 90 miles per hour. What is her approximate average speed for the round trip in miles per hour? Pick
A) 40 B) 45 C) 60 D) 61.7 E) 70
Average speed = total distance/total time   Here distance upstream and back is same as boat follows the same route.   D = 30T1 and D=90T2   Therefore, T1 = D/30 and T2 = D/90   T1+T2 = 4d/90   Therefore, average speed = 2d/4d/90   =2d * 90/4d   Answer is 45   Answer is 45
<result>B</result>
A boat round trip in a river goes upstream at an average speed of 30 miles per hour. On the trip back downstream boat goes at an average speed of 90 miles per hour. What is her approximate average speed for the round trip in miles per hour?