28 miles upstream requires 28/7 = 4 hours.
28 miles downstream requires 28/21 = 4/3 hours, or 1 hour and 20 minutes.
How long does it take him to cover 28 miles?
Apply the distance formula:
d = rt
where
d is distance
r is rate or speed
t is time
21 = t*(r+7) = rt+ 7t
7 = t *(r-7) = rt - 7t
28 = 2rt
14 = rt
21 = rt + 7t
21 = 14 + 7t
7 = 7t
t=1
21 = (r+7)*1 = r+7
r = 14
The speed of the boat is 14 mph. With the current (downstream), the speed is 14+7 = 21 mph, so 21 miles are traveled in 1 hour. Against the current (upstream), the speed is 14-7 = 7 mph, which means 7 miles are traveled in the same 1-hour time frame.
28 miles downstream requires 28/21 = 4/3 hours, or 1 hour and 20 minutes.
28 miles upstream requires 28/7 = 4 hours.
To learn more about downstream refer to:
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