- Thread starter
- #1
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?
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.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?
Now there are 3 minutes remaining in the 7.Start then together, turn the 4 when it is emply.
When the 7 is empty there is 1 minute remaining in the 4. So why would you want to turn it?When the 7 is empty turn the 4.
At this point it has 1 minute to go...
Typo, now corrected (I hope)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?
Okay, that works but it's a little inefficient as it wastes the first 7 minutes. There's a better solution.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
yes +1Hint: There is a solution that wastes no time and measures 9 minutes in one go.