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19 Jan 2013
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Hourglasses 4, 7 Puzzle

Genre: Lateral Thinking Puzzle Rating: 4 - Medium Source: Unknown
Brain Functions: Lateral Thinking / Exact calculations Hint    Solution

You have two hourglasses: one that measures exactly 4 minutes and one that measures exactly 7 minutes. They don't have any markings indicating individual minutes or fractions of minutes (just 4 and 7). You wish to measure exactly 9 minutes. Both hourglasses have been standing upright for a long time. How would you use these two hourglasses to measure 9 minutes?

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