In the hourglass timing puzzle, which step is part of a solution to time exactly 9 minutes?

• A. Start both hourglasses and wait until both finish, then total time.
• B. Let the seven minute run to completion, then stop the four minute hourglass at its halfway point.
• C. Start the seven minute hourglass first, then flip the four-minute after two minutes to measure 9 total.
• D. Run only the four minute hourglass and double the time.

Answer: (D)

The thing being tested is how to build a precise total time by composing intervals you can control with the hourglasses. The most reliable way to hit 9 minutes is to use the shorter timer as the building block and repeat it to accumulate time, then align the final moment so the total reaches 9. Running the four-minute hourglass twice gives a solid 8-minute block that you can reproduce exactly, without depending on the unpredictable alignment of the seven-minute hourglass finishing at a specific moment. This approach is robust because it stays with a single, controllable timing unit and uses repetition to reach the target, rather than trying to force a 9-minute mark from the natural finish timings of both glasses. The other options depend on particular coincidences of when the seven-minute finishes or require mixing the two timers in ways that don’t guarantee a clean 9-minute result.