Braingle: 'Hour Glass' Brain Teaser

Hour Glass

Logic puzzles require you to think. You will have to be logical in your reasoning.

Puzzle ID:	#29696
Fun:	(2.93)
Difficulty:	(2.1)
Category:	Logic
Submitted By:	npf2005

With a 7-minute hourglass and an 11-minute hourglass, can you explain the quickest way to time a boiling egg for 15 minutes?

Answer

Start both hour glasses as the egg is dropped in the boiling water. When the sand stops running in the 7-minute hourglass, turn it over. When the sand stops running in the 11-minute hourglass, turn the 7-minute hourglass over and leave the 11-minute hourglass. When the sand stops in the 7-minute hourglass, 15 minutes will have elapsed.
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Comments

RGW4 Apr 11, 2006	Nice teaser. I know I recall this from my childhood; but its such a logical one that it is fun to re-figure again! Good job, keep them up!
fem81uk Apr 11, 2006	i had something slightly diffferent that also worked but it was on the same principal. good one.
safire2191 Apr 11, 2006	i may be reading it wrong but to me the answer reads 18 minutes. because you essentially wait the 11 minutes and then use the 7 minute timer. i would have started both timers at the same time, put the egg in when the 7 minute timer was done and turned it over again when the 11 minute timer was up.
mercenary007 Apr 11, 2006	nice one... I figured it out quite easily though
npf2005 Apr 12, 2006	it can be done both ways
Atropus Apr 13, 2006	The answer that was given is quicker as you don't have to wait to put the egg in the water.
npf2005 Apr 13, 2006	there is only one answer for THE TEASER. i said quickest way
ahpuppyeyes Apr 14, 2006	I liked it
RRAMMOHAN May 23, 2006	Simple, but fun.
cyberstar5150 Jul 07, 2006	How about this? 1) Set both hourglasses and wait until the 7-min glass expires. Flip the seven-minute glass and keep the 11-min glass (7 min past) 2) When the 11-min glass has expired (which held 4-min worth of sand), flip the 7-min glass (which has dumped 4-min worth of sand) (11 min past) 3) When the 7-min glass expires, 15 minutes have passed.
drift15 Jul 11, 2006	how about this? 1. dont drop the egg into the water yet, first make both hourglasses go at the same time until the 7 minute glass expires(7min expired, 4min remaining in 11min hour glass) 2. now drop the egg in the water and flip over the 11 min(when expired, 4 minutes would have gone by) 3. flip over the 11 min again and when expired it will be 15 min
dalfamnest Sep 09, 2009	Good teaser, thanks. The original answer is clearly the fastest as you start cooking and timing together. Other solutions offered are either slower (drift) or essentially the same as the original (cyberstar).
VitalStatistic Sep 09, 2009	Loved it. Thanks npf2005 I agree, I got 15 minutes, but yours was quicker.
tahoegiles Sep 09, 2009	An awesome teaser. I'm an hourglass collector. Now if only I could find a 7 min. and an 11 min. hourglass! I've got a 90 min. hour glass and a lot of 60 min. hour glasses and gobs of 3 min. ones. But who's got the 7 min?
(user deleted) Sep 09, 2009	All over complicated: 1) start both glasses 2) when the 7 min glass expires put the egg in the water 3) when the 11 min glass expires restart it (the egg will have already been in for 4 min.)
doehead Sep 09, 2009	Simple math.
Jimmymo Sep 09, 2009	Very EZ! But fun though!!
FatHead Sep 09, 2009	I got it, but I had to peek first. And I've even seen it before! Sad. To kleine98, he said the quickest way, not the least complicated. Your way works, but takes 4 min. extra.
mattstoc Sep 09, 2009	I felt like I needed to be in Die Hard with a vengence
UptheHill Sep 09, 2009	NICE1, keep them coming!!!
leroyleroy Sep 09, 2009	WELL WELL,, IVE BEEN COOKING FOR 40 YRS AND DID NOT KNOW YOU CAN PUT EGGS IN BOILING WATER.. I WILL SOON TRY IT AND SEE WHAT HAPPENS. EGGS ARE EGGCEPTIONAL.
dsjt Sep 09, 2009	I agree with safire2191. The "correct" answer seems to give 18 minutes not 15. You run the two timers together for 7 minutes. Then you let the 11 minute timer run till it stops. SO add another 4 minutes. Then you run the 7 minute timer again for another 7 minutes. I get 7 + 4 + 7 = 18. The only way to get it to work is to put the egg in after the 7 minute timer stops. Then you put the egg in and let the 11 minute timer run for the remaining 4 minutes. Then turn the 11 minute timer over for the full 11 minutes. 4 + 11 = 15. How does the "correct" answer give you 15 minutes?
dsjt Sep 09, 2009	I got it now. The 7 minute timer is turned over after the first 7 minutes Is allowed to run for 4 more minutes. Then is turned over a final time to let the 4 minutes run back out. 7 + 4 + 4 = 15. Gotcha.
sumZ Sep 09, 2009	nice teaser i got the answer too. but thanks dsjt, your explanation of answer is much easier to understand
2thGirl Sep 10, 2009	WAAYYY too tired to try this one; should've waited till I was more awake!
jetmd Sep 10, 2009	Sorry, But the original answer is simply INCORRECT. The total only adds up to 14 mins. Not 15 mins. kleine98 seems to have the best solution to the problem.
npf2005 Feb 04, 2010	Jetmd: first of all, the original answer is not incorrect: 7 minutes elapses (pretty straight forward). Then, once the 11 minute hourglass finishes, the 7 minute hourglass has been going for 7 minutes and 4 minutes (totalling the 11 minutes of the other hourglass). This means the 7 minute hourglass when turned over will take 4 minutes again to finish (the same amount of sand needs to pass through again), thus totalling 15 minutes. Make sure your math is correct before you critisize ;) Also, the people who have said 1) start both glasses 2) when the 7 min glass expires put the egg in the water 3) when the 11 min glass expires restart it (the egg will have already been in for 4 min.) are also correct
Babe Sep 09, 2012	I find that the quickest way to boil an egg for 15 minutes is to set the timer on the stove.
mom2boys Sep 09, 2012	All answers are equally correct. No combination of 15 minutes is 'quicker' than another combination of 15 minutes. Now if the question is about the fewest number of steps, that's different. Overall, I'm not a fan of teasers that are copied from other sources.
cutebug Sep 09, 2012	Break the raw egg into a glass of beer. You don't have to worry about timing and you can enjoy the beer.
iggy39 Sep 09, 2012	I would just use the timer on my microwave but that wouldn't be a teaser then would it. Never get these things.
spikethru4 Sep 09, 2012	@mom2boys and others: the 'second' method takes 22 minutes - although the egg is only boiling for 15 - and is therefore not the quickest method.
TheWhizKid Oct 14, 2012	Couldn't think of one...