# sand watch

hello everyone try this, if we have 2 sand watches of 11 and 7 minutes respectively,how can we determine 15 minutes time using these two sand watches only????( you can't use your watch,assume that you don't have any watch without these two,you are a man of ancient era )

Note by Eraz Ahmed
5 years, 4 months ago

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Hi! The answer to this question: Start both the sand watches. When the 7 minutes sand watch runs out, turn it over (seven minutes elapsed). When the 11 minute sand watch runs out, turn it over (eleven minutes elapsed). This time when the 7 minute sand watch runs out, the 11 minute sand watch has been running for 4 minutes, turn it over. When this sand watch runs out, 15 minutes have elapsed.

- 5 years, 4 months ago

That's not right.It should be: Start both sand watches.When the 7 minute sand watch runs out,turn it over(time elapsed:7 min).When the 11 minute sand watch runs out(time elapsed:11 min),turn the 7 minute sand watch over (4 minutes have elapsed since the 7 minute sand watch was turned).When this sand watch runs out,15 minutes is over. In your solution,the total elapsed time is 17 minutes.

- 5 years, 4 months ago