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December 1, 2010

Hourglass Puzzle

You have 2 hourglasses, a 7 minute hourglass and an 11 minute
hourglass, what is the quickest way to time the boiling of
an egg for 15 mins?


Avishek said...

pretty simple start with both hour glass at a time. Now the 7min hr glass will complete first. when 7min completes first...turn it upside down immediately until 11min hour glass finishes...now when 11 min hr glass finished turn 7min hr glass upside down (between 7 to 11 sand measuring 4 min will be deposited under 7min hr glass) when 7min finishes you have spent 15 min :)

Anonymous said...

Start with both hour glasses at the same time. After sand trickles down completely from 7 minute hourglass start boiling the egg. In 4 minutes, 11 minute glass will be empty. Start with 11 minute glass again. When it is empty, the egg will have been boiled for 15 minutes.

Secret Squïrrel said...

@Anon - yes, but it took you a total of 22 mins to do that. Avishek's method requires 15 mins *total*.

Anonymous said...

11 - 7 = 4

7 - 4 = 3

11 - 3 = 8

7 + 8 = 15 min

Anonymous said...

You all are so so so silly. Forget about the 11 minute one.
Start the 7 minute hourglass. Once it has finished turn it over and start again. 14 minutes. Then count to 60. DUHHHHHHHHHH!!!!!!!!!!

Anthony Fontana said...

run the 11 and then the 2 twice.

SpottyMo said...

Turn both 7 and 11 mins hourglass together . When 7 mins hourglass finishes , 11mins hourglass left with 4mins.Then you start boiling , so after 4mins finishes, flip the 11mins houglass again . Wait till it finishes.Only flipped twice. ^^
4 + 11 = 15
Tada! ^^

Anonymous said...

use a stopwatch

Anonymous said...

I second SpottyMo. The puzzle asked for the quickest way. and this is it.
start both the 7min and 11 min hourglass together. start boiling when the 7 min one runs out. reverse the 11 min one to get the remaining 4 min. flip it again to get 11 min. total = (11-7)+11=15.