Math Puzzles and Games
9. Having 2 sand-glasses: one 7-minute and
the second one 4-minute. How can you time correctly 9 minutes.
Solution: Turn both sand-glasses. After 4 minutes turn upside
down the 4-min sand-glass. When the 7-min sand-glass spills the
last grain, turn the 7-min upside down. Then you have 1 minute
in the 4-min sand-glass left and after spilling everything, in
the 7-min sand-glass there will be 1 minute of sand down
(already spilt). Turn the 7-min sand-glass upside down and let
the 1 minute go back. And that's it. 4+3+1+1
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