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One minute from a 3 and a 5 minute glass

hourglassestimingmeasuring

You have a 3-minute hourglass and a 5-minute hourglass and nothing else for measuring time.

Mark out an interval of exactly one minute. The interval need not begin when you start; it just needs a clearly recognisable start and end. Then find the earliest moment at which a one-minute interval can begin.

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Start both glasses together. Watch what the 3-glass looks like when the 5-glass runs out.

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