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More ways to get Braingle...

Checkerboard Chances

Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.

 

Puzzle ID:#42219
Fun:*** (2.3)
Difficulty:*** (2.87)
Category:Probability
Submitted By:billy314*us****!!

 

 

 



Timothy has a checkerboard with ten rows and ten columns and a spinner with the numbers 1, 2, and 3. There is an equal chance of each number being spun. He first spins the spinner and moves a checker from the bottom-left corner up the number of spaces spun. He then spins the spinner again, this time moving the checker right the number of spaces spun. If he repeats this process twice more, what are the chances the checker will land on the same color as the square on which it started?





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Comments

tpg76us**
Jun 15, 2008

I found this to be a very challenging and interesting teaser - good job! I was on the right track but didn't use the "sums", so it got complicated quickly and I didn't get the right answer. I should have had some "humble pie" (I did, anyway, in the end!) and used the hint.
Thanks!
kunjuAin*
Jun 16, 2008

hard but interesting
sourdough
Jun 19, 2008

Mathematically imperative that the answer must come true!

$D
javaguru*us*
Dec 08, 2008

Like all probability problems, there's more than one way to get the answer. I used the principle that I need an even number of odd spins in order to move an even number of spaces. This means that there can be either 0, 2, 4 or 6 odd spins.

There are 6P2 = 15 ways to order either two evens and four odds or four odds and two evens.

This gives:

(1/3)^6 + 15x(1/3)^4x(2/3)^2 + 15x(1/3)^2x(2/3)^4 + (2/3)^6
= 1/729 + 60/729 + 240/729 + 64/729
= 365/729



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