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Measuring Time with Hourglasses 2

Easy
Created: February 1, 2026

Previous puzzle in this series

Easy

Measuring Time with Hourglasses 1

You’re making the perfect soft-boiled eggs: all you have to do is boil eggs in a pot of water for exactly 9 minutes. However, your kitchen timer is broken, and...

After your culinary experience with the soft-boiled eggs, you want to try your hand at cooking a steamed vegetable mix. This time, you need to steam your vegetables for 15 exactly minutes. You still haven’t replaced your broken kitchen timer, but you at least you have your trusty hourglasses.

This time, you have two hourglasses: one measures 7 minutes, the other measures 11 minutes. Using only these two hourglasses, how can you measure exactly 15 minutes?

Hint

Try solving Measuring Time with Hourglasses 1 first.

Solution

Start both hourglasses at the same time.

7-min hourglass: 7 minutes left
11-min hourglass: 11 minutes left
Elapsed: 0 min

At 7 minutes, the 7-minute hourglass runs out. Flip it.

7-min hourglass: 7 minutes left
11-min hourglass: 4 minutes left
Elapsed: 7 min

At 11 minutes, the 11-minute hourglass runs out. Flip the 7-minute hourglass.

7-min hourglass: 4 minutes left
11-min hourglass: done
Elapsed: 11 min

The 7-minute hourglass had been running for 4 minutes since its last flip, so 4 minutes of sand have collected in the bottom. Flipping it means those 4 minutes of sand now run back through “in reverse,” giving us exactly 4 more minutes.

7-min hourglass: done
11-min hourglass: done
Elapsed: 15 min

The 7-minute hourglass finishes its remaining 4 minutes of sand, and we’ve measured exactly 15 minutes.

Next puzzle in this series

Easy

Measuring Time with Hourglasses 3

As your cooking skills improve, you decide to try preparing baked salmon fillet. It needs to go into the oven for exactly 20 minutes. As before, you have two...

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