You have two hourglasses. One empties in 7 minutes. The other empties in 4 minutes. How can you use these two hourglasses to measure exactly 9 minutes?
Solution
Flip both timers over. When the 4-min timer is empty, flip it over (the 7-min timer has 3 minutes left in the top). When the 7-min timer is empty, flip it over (there is now 1 minute left in the top of the 4-min timer). When the 4-min timer is empty for the second time, the 7-min timer has 1 minute worth of sand in the bottom half. Flip it over so there is now 1 minute in the top. When the 7-min timer empties, 9 minutes have elapsed.
| Step | Elapsed Time | 4 Min Timer | 7 Min Timer |
| 1 | 0:00 | FLIP | FLIP |
| 2 | 0:04 | FLIP | 3 min left |
| 3 | 0:07 | 1 min left | FLIP |
| 4 | 0:08 | FLIP (1 min now in top) | |
| 5 | 0:09 | EMPTY |
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