Start both hourglasses at the same time. When the 7-minute hourglass runs out, turn it over. Then when the 11-minute hourglass runs out, turn the 7-minute one over again. By the time it's run out, the 15 minutes will be up.
Here's why: When the 7HG is up, it would remain 8 minutes out of the 15 minutes set. However, between the two HGs, there's a 4-minute difference. By the time the 11HG has run out, 4 minutes would have elapsed... which is when the 7HG would be turned over. The point of when the 11HG has run out to the point when the 7HG runs out after the second turnover, would be another 4 minutes. So, 7 minutes of the HG + 4 minutes as at when the 11HG ran out + 4 minutes from the point 11HG ran out till the point 7HG ran out (i.e. 7+4+4) = 15minutes. (This second part is just the way I understood the teaser).
Start both the two hourglasses at the same time,then when the 7min hourglass run out turn it around. 4 minutes later, the 11min hourglass ran out, turn the 7min hourglass around again, so the 4 min run back again. 11 + 4 = 15 minutes and the test is done
Start both hourglasses at the same time.
ReplyDeleteWhen the 7-minute hourglass runs out, turn it over.
Then when the 11-minute hourglass runs out, turn the 7-minute one over again.
By the time it's run out, the 15 minutes will be up.
Here's why:
When the 7HG is up, it would remain 8 minutes out of the 15 minutes set. However, between the two HGs, there's a 4-minute difference.
By the time the 11HG has run out, 4 minutes would have elapsed... which is when the 7HG would be turned over. The point of when the 11HG has run out to the point when the 7HG runs out after the second turnover, would be another 4 minutes.
So, 7 minutes of the HG + 4 minutes as at when the 11HG ran out + 4 minutes from the point 11HG ran out till the point 7HG ran out (i.e. 7+4+4) = 15minutes.
(This second part is just the way I understood the teaser).
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ReplyDeleteStart both the two hourglasses at the same time,then when the 7min hourglass run out turn it around. 4 minutes later, the 11min hourglass ran out, turn the 7min hourglass around again, so the 4 min run back again. 11 + 4 = 15 minutes and the test is done
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