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November 5th, 2013, 08:53 AM   #21
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From: Ottawa Ontario, Canada

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Re: Probability help

Quote:
Originally Posted by Pero
Quote:
 Originally Posted by Denis Take 4 kids only (n=4): 1 1 2 2 * 1 2 1 2 1 2 2 1 2 1 1 2 2 1 2 1 2 2 1 1 * Clearly last 2 kids get same 1/3 of time. But your formula gives: 1 - (1/2)^2 * C(2,1) = 1/2 What am I missing?
These events are not equally likely. Note that the coin is only tossed 2-3 times.

1122 has probability of 1/4 *
1212 has probability of 1/8
1221 has probability of 1/8
2112 has probability of 1/8
2121 has probability of 1/8
2211 has probability of 1/4 *
Ah geezzzz...of course...keep forgeting a coin is being flipped!
Thanks again.
Methinks I'm now ready to assist in handing out burgers to kids

 November 5th, 2013, 09:29 AM #22 Newbie   Joined: Nov 2013 Posts: 8 Thanks: 0 Re: Probability help If p1 = 1 - p2, then it's opposite event, so the two last kids NOT getting the same type? Because in your first post, p1 meant two last kids getting same burger in general, and p2 meant two last kids getting the same burger after 18 coin flips. So the final formula for the problem is: p = 1 - ( (n-2)C(n/2-1) / 2^(n-2) ) p.s: how do I write formulas in graphical form, as Pero did? I checked address of the image, and it has the formula in it which I can edit and it will generate new picture. How can I do this inside posts?
November 5th, 2013, 12:04 PM   #23
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From: London, England

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Re: Probability help

Quote:
 Originally Posted by Jack37 If p1 = 1 - p2, then it's opposite event, so the two last kids NOT getting the same type? Because in your first post, p1 meant two last kids getting same burger in general, and p2 meant two last kids getting the same burger after 18 coin flips.
Just because p1 = 1 - p2 doesn't mean that they are complementary events. The last two kids getting different burgers is the complementary event. But, the probability of this is the same as p2

Quote:
 So the final formula for the problem is: p = 1 - ( (n-2)C(n/2-1) / 2^(n-2) )
No. It's $\frac{1}{2^{n-2}}\binom{n-2}{\frac{n}{2} - 1}$

Quote:
 p.s: how do I write formulas in graphical form, as Pero did? I checked address of the image, and it has the formula in it which I can edit and it will generate new picture. How can I do this inside posts?
Use Latex. Look at other posts to get the syntax and try here, for example, for reference:

http://www.maths.tcd.ie/~dwilkins/LaTeXPrimer/

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