You have two hourglasses: a 7-minute and an 11-minute one. How can you measure exactly 15 minutes using both?
Category: Math · Difficulty: medium
Hint: Use the time that each hourglass runs out as a trigger.
Answer
Start both at the same time, flip the 7-minute when it runs out, then flip the 11-minute when it runs out.
Explanation
When the 7-minute runs out, flip it. When the 11-minute runs out, 4 minutes have passed on the 7-minute. Flip it again, and when it runs out, 15 minutes will have elapsed.
This is a medium math riddle — a moderate challenge for most people. The answer is Start both at the same time, flip the 7-minute when it runs out, then flip the 11-minute when it runs out..
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