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 minutes–without the process taking longer than nine minutes

Here is my solution:

Last updated: 09.09.2017,
created: 29.12.2011.