Question

A motorboat with the current went 72 mi in 4 hours. Against the current, the motorboat could go

only 40 mi in the same amount of time. Find the rate of the motorboat in calm water.

Answer 1

14 m/hr

Explanation:

72 m / 4 hr = 18 m/hr

40 m / 4 hr = 10 m/hr

still water speed = (18+ 10 )/2 = 14 m/hr

current speed (14 -x) 4 = 40

x = current speed = 4 m/hr

Answer 2

14 mph

Explanation:

Speed of boat in calm water = x

Speed of current = y

Speed of boat with the current = 72 mi / 4 hr = 18 mph

Speed of boat against current = 40 mi / 4 hr = 10 mph

x + y = 18

x - y = 10

2x = 28

x = 14

Answer: 14 mph