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Hour Glass

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


Puzzle ID:#29696
Fun:*** (2.93)
Difficulty:** (2.09)
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?

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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!
Apr 11, 2006

i had something slightly diffferent that also worked but it was on the same principal. good one.
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.
Apr 11, 2006

nice one... I figured it out quite easily though
Apr 12, 2006

it can be done both ways
Apr 13, 2006

The answer that was given is quicker as you don't have to wait to put the egg in the water.
Apr 13, 2006

there is only one answer for THE TEASER. i said quickest way
Apr 14, 2006

I liked it
May 23, 2006

Simple, but fun.
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.
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
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).
Sep 09, 2009

Loved it. Thanks npf2005
I agree, I got 15 minutes, but yours was quicker.
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.)
Sep 09, 2009

Simple math.
Sep 09, 2009

Very EZ! But fun though!!
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.
Sep 09, 2009

I felt like I needed to be in Die Hard with a vengence
Sep 09, 2009

NICE1, keep them coming!!!
Sep 09, 2009

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?
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.

Sep 09, 2009

nice teaser i got the answer too.
but thanks dsjt,
your explanation of answer is much easier to understand
Sep 10, 2009

WAAYYY too tired to try this one; should've waited till I was more awake!
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.
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
Sep 09, 2012

I find that the quickest way to boil an egg for 15 minutes is to set the timer on the stove.

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.
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.
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.
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.
Oct 14, 2012

Couldn't think of one...
Sep 09, 2015

I read it again without knowing I had done it before and I said the same answer again as I thought it was a trick question. Actually it is a great answer and does not take any figuring or messing with sand.
Sep 09, 2015

A chef's opinion:
You don't hard boil an egg by dropping it into boiling water. You put the egg in cold water and then start the stove...and since it only takes 11 minutes to hard boil an egg properly, all you need to do is turn over the 11 minute hourglass.
Sep 09, 2015

...and the alternate solution might be considered 'quicker' if that means takes less time to accomplish the manipulation. The 1st answer is better only if 'quicker' refers to total elapsed time!

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