It took 3 hours to row a boat 18 km against the current. The return trip with the current took 1 1/2 hours. How do you find the speed of the rowboat in still water?

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Answer 1 · 2015-10-28T22:52:52

The speed is 9 km/h.

Boat speed = Vb
River speed = Vr

If it took 3 hours to cover 18km, the average speed #=18/3=6# km/h

For the return trip, the average speed is #=18/1.5=12# km/h

#{(Vb-Vr=6),(Vb+Vr=12):}#

According to the second equation,

#Vr=12-Vb#

Substituting in the first equation:

#Vb-(12-Vb)=6)#

#Vb-12+Vb=6#

#2Vb=6+12#

#Vb=18/2 = 9#