hardmath

You want to boil a two-minute egg. If you only have a three-minute timer (hourglass), a four-minute timer and a five-minute timer can you boil the egg for only two minutes?

Once the water is boiling, turn the three-minute timer and five-minute timer over. When the three-minute timer runs out, put the egg in the boiling water. When the five-minute timer runs out, two minutes have elapsed and it is time take the egg out of the water. You don't need the four-minute timer for this riddle.

Why this works

This riddle involves strategic timing with the available hourglasses to measure out exactly two minutes. By flipping the three-minute and five-minute timers simultaneously when the water starts boiling, you create a situation where the three-minute timer counts down while the five-minute timer continues to run. When the three-minute timer runs out, you know exactly when to start boiling the egg; then, when the five-minute timer runs out two minutes later, you can confidently take the egg out, having boiled it for precisely the required time. The clever use of the different timer lengths allows you to isolate the two-minute interval needed without directly using a two-minute timer.

Common Wrong Answers

“Use the four-minute timer to measure two minutes.”

The four-minute timer cannot directly measure two minutes; it only measures four minutes, which is too long for boiling a two-minute egg.

“Start the three-minute timer and then flip it again after it runs out.”

If you flip the three-minute timer after it runs out, you will not accurately measure two minutes since you would be adding more time rather than keeping track of the two-minute boil.

“Use the five-minute timer and just subtract three minutes when it runs out.”

The five-minute timer measures five minutes continuously and does not provide a way to stop and accurately determine when two minutes have passed.

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