Question

The Lazy River has a current of 5 miles per hour. A motorboat can travel 20 miles

down the river in and back in 3 hours. What is the speed of the boat in still water?
Please show your work and use table to relate time, speed, distance.

Answer 1

5 mph

Explanation:

Answer 2

15 mph = rate of boat in still water

Explanation:

D = RT

Let R = rate of boat in still water

Rate down stream = R + 5

Time down stream = 20/(R + 5)

Rate upstream = R - 5

Time upstream = 20/(R - 5)

Total time = 20/(R + 5) + 20/(R - 5) = 3

20(R - 5) + 20(R + 5) = 3(R + 5)(R - 5)

20R - 100 + 20R + 100 = 3R^2 - 75

3R^2 + 40R - 75 = 0

(3R + 5)(R - 15) = 0

R = -5/3 or 15

Rate cannot be negative, so R = 15 mph = rate of boat in still water