Mental Floss 310

The answer to last week’s challenge is: Q: With a 7 minute hourglass and an 11 minute hourglass, what would be the easiest way to tell exactly when 15 minutes were up?

A: Start both hourglasses together. When the sand stops in the 7 minute hourglass, turn it over. When the sand stops in the 11 minute hourglass, turn the 7 minute hourglass over and leave the 11 minute hourglass. When the sand stops in the 7 minute hourglass, 15 minutes will have passed.

This week’s thinking challenge is …

Q: There are 20 people at a party. If each of them is willing to shake hands only with people who are smaller than they are, how many people will shake hands in total?

The answer along with a new challenge, next week.

Have an excellent weekend!

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