A jet boat is used to take canoers 200 miles upriver to their starting point. As the driver of the jet boat, Jeremy knows that it will take him 5 hours to get the canoers upstream, but only 4 hours to get the boat back downstream after dropping the canoers off, as long as he keeps the boat moving at a constant speed.

How fast would the boat be moving in still water?
A.
5 mph
B.
40 mph
C.
45 mph
D.
50 mph

Question

A jet boat is used to take canoers 200 miles upriver to their starting point. As the driver of the jet boat, Jeremy knows that it will take him 5 hours to get the canoers upstream, but only 4 hours to get the boat back downstream after dropping the canoers off, as long as he keeps the boat moving at a constant speed.  

How fast would the boat be moving in still water?
A.
5 mph
B.
40 mph
C.
45 mph
D.
50 mph

ranga · Answer

Effective speed when going upstream is: 200 miles / 5 hours = 40 mph.
Effective speed when going downstream is: 200 miles / 4 hours = 50 mph.
Let u be the speed of the boat and r be the speed of the river.
When going upstream, the effective speed of the boat = u - r = 40
When going downstream, the effective speed of the boat = u + r = 50
Solve for u from these two equations.