Tuesday, December 11, 2007
Timing With Hourglasses - the Answer
(Solution Hidden)
1. Start both hourglasses when you grab the first little piggy.
2. When the 7-minute glass runs out, turn it over.
3. When the 11-minute hourglass runs out, turn the 7-minute glass again.
At this point, 11 minutes have passed and 4 minutes remain in the 7-minute hourglass.
4. When the 7-minute glass runs out, 15 minutes have passed.
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2 comments:
The math doesn't seem to add up:
When the 11 min glass runs out you have 3 min. left in 7 min glass. hence when you turn the 7 min. glass again you will get 14 min (not 15)
________7 min_________7 min
________________11min
11-7=4 min. which means only 3 min is left in the 7 min glass. hence total time is 14min. (seen from above)
We start both timers.
When the 7 min glass finishes, we turn it over.
Now, when the 11 min glass runs out, we have 3 mins left in the 7 min glass, but now we turn the 7 min glass over again.
This means that we now have 4 mins, so we get our 15 minutes.
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