Which step is part of the solution to timing exactly 9 minutes with 7- and 4-minute hourglasses?

• A. Let the seven minute hourglass run to completion and stop the four minute hourglass at its halfway point.
• B. Start the four minute hourglass alone and wait until it finishes.
• C. Start both hourglasses and ignore the results.
• D. Use a digital timer instead of hourglasses.

Answer: (A)

The idea being tested is how to combine two different timers by pausing one to line up a precise total elapsed time. Start both hourglasses at the same moment. Let the four-minute hourglass run for two minutes and then stop it at its halfway point. Now you’ve used two minutes, and the four-minute glass still has two minutes of sand left to run once you restart it. Meanwhile, the seven-minute hourglass keeps running and finishes at seven minutes. At that moment, resume the paused four-minute hourglass; it has two minutes left, so it will finish two minutes later, at nine minutes total. This neatly achieves exactly nine minutes using only the two hourglasses, with a pause to create the necessary offset.

The other options don’t create that precise alignment: starting only the four-minute timer yields four minutes, not nine; starting both and ignoring results doesn’t track time; and using a digital timer isn’t using the hourglasses as the puzzle intends.