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22 October, 11:03

Maria takes a boat upstream for 9 hours. Her return trip downstream only took 6 hours. If the river she is on has a current of 1 mile per hour, how fast does her boat move in still water?

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  1. 22 October, 12:01
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    5 mph

    Explanation:

    Let her speed in still water be v (mph) relative to water

    When moving upstream, her speed relative to ground is v - 1, her time is 9 hours, so her distance traveled is 9 (v - 1)

    When moving downstream, her speed relative ground is v + 1, her time is 6 hours, so her distance is 6 (v + 1)

    Since both distances upstream and downstream are the same, we can equate the 2 above

    9 (v - 1) = 6 (v + 1)

    9v - 9 = 6v + 6

    3v = 6 + 9 = 15

    v = 5

    So her speed in still water is 5 mph
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