How can you measure 9 minutes with a 7-minute hourglass and a 4-minute hourglass?
Note: In this question it states where it's possible or not but not the actual procedure for this exact situation.
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We start the hourglasses at the same time. When the 4-minute hourglass stops we turn it to the other side. The 7minute hourglass will have three more minutes to stop, so when it stops we turn it to the other side and in the meanwhile the 4minute one will have 1 more minute to stop. When the 4minute one stops we turn it to the other side and in the meanwhile the 7minute hourglass will need 6 minutes to stop. When the 4minute hourglass stops the 7minute one will need 2 minutes to stop. We start measuring from this time the 2 minutes to stop and we turn it to the other side for the rest 7 minutes.