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benjamin2 · 2009-07-03 11:45:07

I made up this puzzle for you to work out:

You need to time 7 minutes, but all you have are two hourglasses.

One hourglass measures 3 minutes.
The other measures 5 minutes.
You need EXACTLY 7 minutes.

How should you time the 7 minutes?
Post your solutions!

Last edited by benjamin2 (2009-07-03 12:03:47)

SmartIrishKid · 2009-07-03 12:07:10

1. Start both hourglasses
2. When the 3 runs out, turn it over
3. When the 5 runs out, turn the 3 over
4. When the 3 runs out again, you have measured 7 minutes.

My dad helped me big_smile

Last edited by SmartIrishKid (2009-07-03 12:07:43)

demosthenes · 2009-07-03 12:07:33

Flip them both at the same time wait until the 3 minute expires, two minutes left on the other one. Wait for it to expire and then flip it! 7 minutes! smile

spacetime · 2009-07-23 02:24:45

In general you can measure any time that is the sum of individual hourglasses times and their differences.

Blade-Edge · 2009-07-23 02:39:01

throw the 3 second hourglass at someone's face
then count to 2 and flip the 5 second one
smile

technoguyx · 2009-07-23 10:12:30

Blade-Edge wrote:
throw the 3 second hourglass at someone's face
then count to 2 and flip the 5 second one

Actually, it's MINUTES, not seconds. So, count to 120. tongue

Blade-Edge · 2009-07-23 15:29:37

uh whoops
thought it was 7 seconds xD

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archived forums

#1 2009-07-03 11:45:07

7 minute hourglass problem

#2 2009-07-03 12:07:10

Re: 7 minute hourglass problem

#3 2009-07-03 12:07:33

Re: 7 minute hourglass problem

#4 2009-07-23 02:24:45

Re: 7 minute hourglass problem

#5 2009-07-23 02:39:01

Re: 7 minute hourglass problem

#6 2009-07-23 10:12:30

Re: 7 minute hourglass problem

Blade-Edge wrote:

#7 2009-07-23 15:29:37

Re: 7 minute hourglass problem

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