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Hourglasses

Alexmahone

Active member
Jan 26, 2012
268
Bob owns a couple of hourglasses -- one is a 4 minute hourglass and the other is a 7 minute hourglass. If Bob wants to measure 9 minutes, how could he do it?
 

CaptainBlack

Well-known member
Jan 26, 2012
890
Bob owns a couple of hourglasses -- one is a 4 minute hourglass and the other is a 7 minute hourglass. If Bob wants to measure 9 minutes, how could he do it?
Start then together, turn the 4 when it is emply. When the 7 is empty start timing the nine minutes, at this point the four minute glass has 1 minute to go. When empty turn it, 5 minutes have elapsed. Turn it again, and when empty 9 minutes have elapled.

CB
 
Last edited:

Alexmahone

Active member
Jan 26, 2012
268
Last edited by a moderator:

CaptainBlack

Well-known member
Jan 26, 2012
890
Now there are 3 minutes remaining in the 7.



When the 7 is empty there is 1 minute remaining in the 4. So why would you want to turn it?
Typo, now corrected (I hope)

CB
 

Alexmahone

Active member
Jan 26, 2012
268
Start then together, turn the 4 when it is emply. When the 7 is empty start timing the nine minutes, at this point the four minute glass has 1 minute to go. When empty turn it, 5 minutes have elapsed. Turn it again, and when empty 9 minutes have elapled.

CB
Okay, that works but it's a little inefficient as it wastes the first 7 minutes. There's a better solution.
 

Alexmahone

Active member
Jan 26, 2012
268
Hint: There is a solution that wastes no time and measures 9 minutes in one go.
 

conscipost

Member
Jan 26, 2012
39
Start the two together. When the four is empty, flip it (four minutes elapsed). When the seven is empty, flip it (7 minutes). When the four is empty again flip the seven (8 minutes). Wait then the remaining minute for the seven to empty.
 
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oasi

New member
Mar 14, 2012
14