Interesting hour glass puzzle

[ujjagrawal]

You are a cooking in remote area with no clocks or other way of keeping time other than a four-minute and a seven-minute hourglass. On the stove is a pot of boiling water. Your spouse asks you for a nine-minute egg, and you know she/he is a perfectionist and can tell if you under-cooked or overcooked the egg by even a few seconds. How can you cook the egg for exactly 9 minutes?

[Potok]

Being an hourglass perfectionist..

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go for 7+2, stop the 2nd hourglass when its on half.

[MikeD]

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Start both hour glasses:

When the four minute one runs out, turn it.

When the seven minute one runs out, there should be one minute left of the four minute one.

Put the egg in.

When the four minute runs out, turn it (egg cooked for one minute)

When the four minute runs out again turn (egg cooked for (1+4) minutes

When the four minute runs out again turn (egg cooked for (1+4+4) minutes

Serve a perfectly nine-minute cooked egg.

[hhh3]

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put egg in. to start with u have (4,0) and (7,0) after 4 mins it will be (0,4) and (3,4). invert 4 min one. then after 7 mins, u'll get (1,3) and (0,7). now invert 7 min one. so after 8 min u get (0,4) and (1,6). agian invert and finally u get ur 9 mins

well.. in my mind it was all too simple.. but puttin it down was real hard for me... all jst to get a boiled egg ?????

[hhh3]

  On 6/20/2012 at 6:27 AM, MikeD said:

  Reveal hidden contents

Start both hour glasses:

When the four minute one runs out, turn it.

When the seven minute one runs out, there should be one minute left of the four minute one.

Put the egg in.

When the four minute runs out, turn it (egg cooked for one minute)

When the four minute runs out again turn (egg cooked for (1+4) minutes

When the four minute runs out again turn (egg cooked for (1+4+4) minutes

Serve a perfectly nine-minute cooked egg.

nicely done.. mine looks like a set of coordinates... even i cant really understand it now let alone any1 who might read it..

[ujjagrawal]

  On 6/20/2012 at 7:15 AM, hhh3 said:

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put egg in. to start with u have (4,0) and (7,0) after 4 mins it will be (0,4) and (3,4). invert 4 min one. then after 7 mins, u'll get (1,3) and (0,7). now invert 7 min one. so after 8 min u get (0,4) and (1,6). agian invert and finally u get ur 9 mins

well.. in my mind it was all too simple.. but puttin it down was real hard for me... all jst to get a boiled egg ?????

nice job hhh3...

[ujjagrawal]

  On 6/20/2012 at 6:27 AM, MikeD said:

  Reveal hidden contents

Start both hour glasses:

When the four minute one runs out, turn it.

When the seven minute one runs out, there should be one minute left of the four minute one.

Put the egg in.

When the four minute runs out, turn it (egg cooked for one minute)

When the four minute runs out again turn (egg cooked for (1+4) minutes

When the four minute runs out again turn (egg cooked for (1+4+4) minutes

Serve a perfectly nine-minute cooked egg.

MikeD... your method will let your spouse wait for 16 mins to get an 9 mins boiled egg... I will like you to deliver the same just after 9 mins and I know you can do it ...

[ujjagrawal]

  On 6/20/2012 at 5:06 AM, Potok said:

Being an hourglass perfectionist..

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go for 7+2, stop the 2nd hourglass when its on half.

that's smart.... but will like you to try harder on it... otherwise a time perfectionist then even won't need any hour glasses.

[Half Fast]

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Too Simple Start both timers, when 4 is done, flip it over. 4 min passed, 7 timer has 3 min left. When 7 timer is done, flip both over. 7 min passed, 4 timer has 1 min left, 7 timer full. When 4 timer is done, flip 7 timer over. 8 min passed and 7 timer has one min in bottom which is why you flip it to time the last min.

[Rox53]

I must have started reading this topic too late. I got the same answer as Half Fast after a couple of attempts. But I wasn't as devious as Mike D, who had to wait an extra 4 minutes.

Who boils their eggs for 9 minutes anyway? It must be very hard. I like my yolk runny!

[Peekay]

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Start both hourglasses at the same time.

When 4 is done, turn the 4 - elapsed time 4mins

When 7 is done, turn both (1min left in 4) - elapsed time 7mins

When 4 is done, turn 7 (1min in 7) - elapsed time 8 mins

When 7 is done, your egg is cooked to perfection!

[ujjagrawal]

Some of you had already got the answer... however putting solution for others...

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1. Flip both hourglasses over and drop the egg into the water.

2. When the 4-minute timer runs out, flip it over (4 minutes elapsed, 3 remaining on the 7-minute timer).

3. When the 7-minute timer runs out, flip it over. (7 minutes elapsed, 1 remaining in the 4-minute timer)

4. When the 4-minute timer runs out, flip the 7-minute timer over. (8 minutes elapsed. 6 minutes remained in the 7-minute timer, but flipping it over leaves one minute's worth of sand on top. When it runs out exactly nine minutes will have elapsed.