BY boat Joseph can go 72 km upstream to a fishing hole in 4

BY boat, Joseph can go 72 km upstream to a fishing hole in 4 hours. To go back he needs only 3 hours. What is his rate upstream and his rate downstream?

Solution

Upstream DATA;
distance = 72 km ; time = 4 hrs ; rate = 72/4 = 18 kph
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Downstream DATA:
distance = 72 km ; time = 3 hrs ; rate = 72/3 = 24 kph
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Let b = boat speed ; Let c = current speed
EQUATIONS:
b+c = 24
b-c = 18
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Add to get:
2b = 42
b = 21 (boat speed in still water)
Substitute into b+c = 24 to get:
21 + c = 24
c = 3 kph (current speed)
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Upstream speed = 18 kph
Downstream speed = 24 kph