How to measure 9 minutes using only a 4 minute and 7 minute
hourglass?
Answer: This question is similar to 4 Quarts of Water. With water, we were able to fill and pour out the pail multiple times. We can use the same logic and easily come up solutions which start measuring 9 minutes somewhere in the middle of the procedure but those will not be ideal. If possible, we want to measure the 9 minutes right from the start.
| Step | Time | 4 minute timer | 7 minute timer |
| 1 | 0 min | Start | Start |
| 2 | 4 mins | Flip | 3 minutes left |
| 3 | 7 mins | 1 minute left | Flip |
| 4 | 8 mins | Stop | Flip (1 minute left) |
| 5 | 9 mins | Stop |
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