This time, there were a number of people able to “engineer” a 9-minute period for this Hourglass problem.
However, only a few of you managed to capture the spirit of finding the most elegant solution. Well done!
Start by turning both hourglasses over together.
As soon as the 4-minute glass runs out, you turn it over again [four minutes have elapsed]. When the 7-minute glass runs out, you also turn it over [seven minutes have elapsed].
When the 4-minute glass runs out again for the second time [eight minutes have elapsed]; and the 7-minute glass has now only been running for one minute.
Therefore, you turn it over once again. And when it stops, you then have your total of nine minutes.
You’ll find November’s Mind Bender will test your tennis skills.