Question

A motorboat travels 30 min upstream against a 4-mph current. Returning with current takes 18 min. Find the speed of the motorboat in still water ( watch out the units in motion,One is 30 min but current is mile per hour (mph))

Answer 1

We have, 30 minutes = 30/60 hour = 0.5 hour

And, 18 minutes = 18/60 hour = 0.3 hour

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Now let the speed of the motorboat in still water is x mph

So, speed of the motorboat in upstream is = (x - 4) mph

And, speed of the motorboat in downstream is = (x + 4) mph

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Now, as distance = speed × time, then we have,

0.5(x - 4) = 0.3(x + 4)

i.e. 0.5x - 2 = 0.3x + 1.2

i.e. 0.5x - 0.3x = 1.2 + 2

i.e. 0.2x = 3.2

i.e. x = 3.2/0.2

i.e. x = 16

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So speed of the motorboat in still water is 16 mph