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A boat travels 30 km/h in still water. If this boat goes upstream in a river flowing at a rate of 12 km/h, for 5 km,
and returns back downstream to its starting point, what is the average speed for the whole trip in km/h?
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The boat's own speed in still water is 30 km/h.
The rate of the current in the river is 12 km/h.
So, the boat' speed with the current (downstream) is u = 30 + 12 = 42 km/h;
the boat' speed against the current (upstream) is v = 30 - 12 = 18 km/h.
Now, if you know the theory, you may use the ready formula for the average speed of the two-ways trip
= = = = = 25.2 km/h. ANSWER
If you don't know this theory,
you may complete the solution in couple of steps.
The travel time upstream is of an hour; travel time downstream is of an hour.
The average speed is the total distance 5 + 5 = 10 km, divided by the total time + of an hour.
Thus the average speed is
= = = = the same formula and the seme result = 25.2 km/h.
ANSWER. The average speed of the two-ways trip is 25.2 km.h
Solved.