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        riddles >> easy >> Very easy: 3+4=5
        
(Message started by: BNC on Mar 26^th, 2003, 5:07am)

Title: Very easy: 3+4=5
Post by BNC on Mar 26^th, 2003, 5:07am

I have 2 hourglasses: one measures 3 minutes, the other measures 4 minutes (both unturned).

How can I measure 5 minutes, starting..... now!

Title: Re: Very easy: 3+4=5
Post by cho on Mar 26^th, 2003, 6:28am

You sell the hourglasses and buy a cheap watch.
You get a really strong slingshot and calculate the exact amount of force needed to hurl an hourglass into the air so that it takes five minutes to fall back to earth.
You break open an hourglass and count the grains of sand saying "one mississippi, two mississippi," until you've got a pile of 300 grains.
You tie an hourglass to a piece of string and swing it as a metronome.
You stack them one on top of the other and use them as a sundial.

Title: Re: Very easy: 3+4=5
Post by Kitty on Mar 26^th, 2003, 9:26am

I like cho's ideas but this is what i got... [hide]turn both when 3 run out turn 4 over (this is when you start counting) once 4 is finished turn it again and voila, 5 minutes. [/hide]

Title: Re: Very easy: 3+4=5
Post by harpanet on Mar 26^th, 2003, 10:20am

I think that BNC's comment

Quote:

How can I measure 5 minutes, starting..... now!

means that the time measurement starts the moment you turn any of the hour glasses. Here's my take:
[hide]
Flip both: (0 minutes elapsed)
Once the 3 minute glass finishes, flip it (3 minutes elapsed)
Once the 4 minute glass finishes, flip the 3 glass again (4 minutes elapsed)
Once the 3 minute glass finishes - time up (5 minutes elapsed).
[/hide]

Title: Re: Very easy: 3+4=5
Post by maryl on Mar 26^th, 2003, 11:35am

Couldn't you just flip the 3 minute glass and when it's done, flip the 4 min. glass and stop it halfway thru?

Title: Re: Very easy: 3+4=5
Post by prince on Mar 26^th, 2003, 1:16pm

on 03/26/03 at 11:35:09, maryl wrote:

Couldn't you just flip the 3 minute glass and when it's done, flip the 4 min. glass and stop it halfway thru?

Well then why not just flip the 3 minute glass once, then again, and stop it when it's 2/3 through? Usually hourglass riddles work under the assumptions that 1.) You can't determine fractions of sand left in the top part--actually quite reasonable, and 2.) hourgalss flips can be performed without any time elapsing.

Title: Re: Very easy: 3+4=5
Post by Lowe on Mar 27^th, 2003, 12:11pm

How about this - flip both hourglasses at the same time. When the 3 minute runs out, you will have exactly 1min left in the 4 min hourglass, so flip over the 4 min hourglass (with the 1 min of sand left in it) and as soon as it's out flip it over for the remaining 4 minutes - that's your 5 minutes!

Title: harpanet Posted on: Mar 26th,Re: Very easy: 3+4=5
Post by Kitty on Mar 27^th, 2003, 1:01pm

That what i got but like harpanet said
I think that BNC's comment:
How can I measure 5 minutes, starting..... now!
Means start straight away.

Title: Re: Very easy: 3+4=5
Post by Lowe on Mar 27^th, 2003, 2:26pm

You are so right. How about this: start off the same way by flipping both hourglasses at the same time. When the 3 minute hourglass runs out, you have 1 minute left in the 4 min hourglass. Flip the 4 min over and the 3 min over at the same time, letting the minute left in the 4 min run, then the 3 min hourglass will have exactly min of sand in it, so flip that over and let it run out: exactly 5 min! Let me know if I didn't make it clear!

Title: Re: Very easy: 3+4=5
Post by Icarus on Mar 27^th, 2003, 4:52pm

Lowe- You said to turn the 4 minute glass over with the 3 minute glass when it runs out. This leaves 3 minutes in both.

What you meant, as is clear by the statement that the 4 minute glass has 1 minute left, is that the 4 minute glass is left alone.

What I would like to bring to your attention, though, is that this is the same solution harpanet gives above. As is common in these forums, harpanet has hidden his answer. If you will highlight his response above using your mouse, you will see the hidden text. Anytime in these forums you see large open areas, or missing text, try highlighting it.

We tend to hide our answers so that those looking for a few hints but not the whole thing can find them without seeing the full solution.

Title: Re: Very easy: 3+4=5
Post by Lowe on Mar 28^th, 2003, 4:23am

Thanks for the tip! I had no idea you hide answers. :)

Title: Re: Very easy: 3+4=5
Post by Lowe on Mar 28^th, 2003, 4:41am

Ok, here it is again & I'm trying to figure out how to "hide," so my sincere apologies if it doesn't work...

Title: Re: Very easy: 3+4=5
Post by Lowe on Mar 28^th, 2003, 4:49am

3rd time charm:

[hide]
1) flip both at the same time
2) when 3 runs out, flip 4 again (there is 1 minute left to go in 4) while flipping 3 again at the same time
3) when 4 runs out (remember there was only 1 min left in 4), flip 3 (there is 1 min of sand which has passed in 3)
4) when the 1 min of 3 passes, 5 minutes has elapsed
[/hide]

// hide fixed by william wu

Title: Re: Very easy: 3+4=5
Post by Kitty on Mar 28^th, 2003, 8:42am

You need [/hide] to close it.

Title: Re: Very easy: 3+4=5
Post by Icarus on Mar 28^th, 2003, 6:56pm

One other tip. When you preview, the text is not hidden (nor is it hidden in the summary of the thread displayed below the reply window). But if you cannot see the hide tags themselves you are okay. If you can see [/hide] or [hide] in the preview, then something is wrong. Look in the suggestions forum (http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_suggestions) for some other useful information in posting.