Question 552024
A boat traveled for two hours downstream with a 5km/h current. 
Downstream DATA:
time = 2 hrs ; rate = 5 km/h ; distance = 2*5 = 10 km
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The return trip against thr same current took four hours. 
Upstream DATA:
time = 4 hrs ; distance = 10 hrs; rate = d/t = 10/4 = 5/2 km/hr
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Let "b" be boat speed in still water ; Let "c" be current speed.
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Equation:
b + c = 5 km/hr
b - c = 2.5 km/hr
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Find the speed of the boat in still water.

Add and solve for "b":
2b = 7.2 km/hr
b = 3.6 km/hr (rate of the boat in still water)

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Cheers,
Stan H.
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