Measure 9 minutes with 7 and 4 minutes hourglasses

Today, I’ve read a blog about this year oddest job interview questions. Among really odd questions there is actually an interesting one that gave me today about 5 minutes of pleasant thinking and about half of hour drawing the solution which am about to share.

Here is a puzzle: Using only a four-minute hourglass and a seven-minute hourglass, measure exactly nine minuteswithout the process taking longer than nine minutes

Here is my solution:

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