Time exactly 4 minutes with 9 and 13-minute glasses

Question

You need to blanch a batch of vegetables for exactly 4 minutes. The only timers in the kitchen are two hourglasses: one runs for 9 minutes and the other for 13 minutes. Neither is anywhere close to 4 minutes, and you have no other clock.

How do you time exactly 4 minutes?

Accepted Answer

You do not need any flipping at all, just start both glasses together and watch the gap between when they finish. 1. At time 0, turn over both hourglasses at the same moment. 2. At time 9, the 9-minute glass runs out. Drop the vegetables into the water now. At this instant the 13-minute glass still has 13 - 9 = 4 minutes of sand left to fall. 3. At time 13, the 13-minute glass runs out. Take the vegetables out. The vegetables were in from minute 9 to minute 13, exactly 4 minutes. Why it works. Two hourglasses started together finish 13 - 9 = 4 minutes apart, because the longer one simply keep…

Time exactly 4 minutes with 9 and 13-minute glasses

Your answer

More Brainteasers questions