Having 2 sandglasse...
 
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Having 2 sandglasses: one 7-minute and the second one 4-minute, how can you correctly time 9 minutes?  

 

(@abhiraj-kashyap)
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09/07/2020 11:24 am  

Having 2 sandglasses: one 7-minute and the second one 4-minute, how can you correctly time 9 minutes?


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(@abhiraj-kashyap)
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Joined: 7 months ago
Posts: 78
09/07/2020 11:24 am  

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 = 9


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