Question 370568
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Hi,
Distance = r*t
rowing downstream (with the current) for five hours. Speed =(12kph + xkph)
Distance =(12k/hr + xk/hr)*5hr

Knowing the speed of the current(k/hr), we would know 'exactly' how far  he traveled.  
For example: if the speed of the current was 3k/hr: speed would be (12+3)kph
Distance = 15k/hr*5hr = 75k              

(kilometer ~ .62 mi)