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 )

#MathProblem

Easy Math Editor

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`2 \times 3`	$2 \times 3$
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Comments

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.

Kshitij Johary - 8 years, 3 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.

Tan Li Xuan - 8 years, 3 months ago

Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$