A boat has a speed of 9 miles per hour in calm water. It takes the boat 4 hours to travel upstream but only 2 hours to travel the same distance downstream. Which equation can be used to find c, the speed of the current in miles per hour?
2(9 – c) = 4(9 + c)
9 + c = 4(9 – c)
9 – c = 2(9 + c)
4(9 – c) = 2(9 + c)

Question

Mercury · Answer

for both trips, distance = speed*time

for the upstream trip, the speed of the current is subtracted from the boat speed (the boat is going against the current, so it'll be slower)

upstream speed = (boat speed - current)

conversely, for the downstream trip, the speed of the current is added to the boat speed (the current helps push the boat along downsreadm)

downstream speed = (boat speed + current)

write a distance = speed * time equation for both upstream and downstream; set them equal to each other (distance is the same since the boat is travelling the same distance both times)