May 2018
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Swimmer’s Solution

Michael Phelps, the famous swimmer can swim downstream in a lake in exactly 40 minutes (with the lake current helping his time).
He can swim upstream in the lake in exactly 60 minutes (against the lake current).
The length of the lake is two miles.

How long would it take Michael Phelps to cover the distance of a still lake (no current).

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3 guesses to Swimmer’s Solution

  • Ryver

    48 mins
    ~~~~~
    ~~~~
    Algebra time!
    Let R = Phelps’ speed at still water
    Let S = effect of the stream to his speed

    2 = 40(R+S) = 60(R-S)
    40R + 40S = 60R – 60S
    100S = 20R
    5S = R

    2 = 2/3 of an hour x (R+S)mph
    2 = 2/3 x(6S)
    2 = 4S
    S = 0.5 mph

    R = 2.5 mph

    2/2.5 = 0.8 hours or 48 mins

  • Nailed it, Ryver. Great job!

    Michael Phelps speed = X km/minute
    Lake current speed = Y km/minute

    2/(X+Y) = 402/(X-Y) = 60

    Solving the simultaneous equations gives X = 1/24

    Therefore, to cover 2km will take 2/X = 48 minutes

    You’re today’s winner.