Question

A speed boat went up the Mississippi River at a speed of 40 mph. It then turned around and traveled downstream to the original starting point at a speed of 60 mph. What was the boat’s average speed? (hint: the answer is NOT 50 mph)

Answer 1

let distance between the two stops be d

so

avg. speed for whole journey = total distance/total time

= ( 2d) / ( d/40 + d/50)

= 2/ (1/40+1/50)

= 2 / ( 90/ 200)

= 2× 200/90

= 40/9 mph