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Post by ken on Jul 24, 2009 14:18:37 GMT -5
If you have two hourglasses -- one four-minute timer and one seven-minute timer, how can you measure nine minutes?
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Post by Wormopolis on Jul 25, 2009 22:49:43 GMT -5
0/4 0/7......4/0 4/3......1/3 7/0.....0/4 6/1.....0/4 7/0 ..................4................3..............1.................1
start both hourglasses when the 4 minute one runs out, flip them both when the 7 minute runs out, flip them both when the 4 minute runs out, flip the 7 minute when it runs out, that will be 9 minutes
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Post by ken on Jul 25, 2009 22:55:20 GMT -5
not correct... that is 11
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Post by Wormopolis on Jul 25, 2009 23:09:32 GMT -5
no its 9.
look at the numbers at the top.
if you flip both glasses, and stop the 7 when the 4 runs out, the 7 has 3 minutes left on it.
if you flip the 4 and run it until the 3 minutes runs out, then the 4 has 1 minute left on it, and the 7 is empty.
so far 4 + 3 minutes have passed.
run the 7 minute until the remaining 1 minute has emptied on the 4. the 7 minute has 1 minute on one side, and 6 on the other.
4 + 3 + 1 have passed
run the 1 minute out of the 7
4 + 3 + 1 + 1 = 9 have passed
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Post by ken on Jul 25, 2009 23:22:50 GMT -5
ohh god whatever... The correct answer..
Turn over both hourglasses at once. When the four-minute hourglass runs out, turn it over. When the seven-minute hourglass runs out, turn it over. When the four-minute timer runs out this time (eight minutes have elapsed), the seven-minute hourglass has been running for one minute. Now turn over the seven minute timer back over. When the timer runs out, nine minutes have elapsed.
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