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

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Answer 1 · 2015-03-25T00:29:53

Lets call the speeds $v_{c}$ $v_c$ and $v_{r}$ $v_r$

Then the speed out $= v_{r} - v_{c} = \frac{18}{3} = 6 k m / h$ $=v_r-v_c=18/3=6km//h$

Return trip speed $= v_{r} + v_{c} = \frac{18}{1.5} = 12 k m / h$ $=v_r+v_c=18/1.5=12km//h$

If we add these two equations, the $v_{c}$ $v_c$ 's cancel out:

$2 \cdot v_{r} = 18 \to v_{r} = 9 k m / h$ $2*v_r=18->v_r=9km//h$ and that's your answer

It is easy to see that $v_{c} = 3 k m / h$ $v_c=3km//h$ in case that's the next question.

Answer 2 · 2015-03-25T00:32:14

Average speed = 8km/hr

Total distance = 18 x 2 = 36km

Total time = 3 + 1.5 = 4.5 hr

Average speed = dist/time = 36/4.5 = 8km/hr