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Math Help - Sand-filled timers questions

  1. #1
    Senior Member
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    Sand-filled timers questions

    Could anyone help me explain this to a child aged 9, e.g. very simply? Many thanks...

    If you have two sand-filled timers, one for 11 minutes and one for 7 minutes, how would you use them to time 15 minutes without wasting any time at the start?

    If you start them at the same time, you would end up with one with 4 minutes left in it, but how would that work? Any help, explanations would be greatly appreciated (remember simple please)
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  2. #2
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    Re: Sand-filled timers questions

    Start both timers at the same time. Once the 7-minute timer is empty, flip it. Four minutes later, the 11 minute timer will be empty, but only 4 minutes of sand will have emptied from the 7-minute timer. So, if you flip the seven minute timer just as the 11-minute timer empties, it will only have 4 minutes of sand in the top. That is a total of 11+4 = 15 minutes.
    Thanks from Natasha1 and LimpSpider
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