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A domino is a rectangular tile composed of two squares. An integer is represented on both squares, and each integer 0-9 is paired with every integer 0-9 exactly once to form a complete set. A \(double\) is a domino that has the same integer on both of its squares. What is the probability that a domino randomly selected from a set will be a \(double\)? Express your answer as a common fraction.

 Oct 21, 2019
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The number of dominoes  in a complete set  where n is the largest integer in the set  is

 

(n + 1) (n + 2)  / 2

 

So....we have

 

(9 + 1) (9 + 2) / 2  =

 

(10)(11)/ 2  =    

 

110/2 =

 

55

 

10 of these dominoes will be "doubles"

 

So....the probability of a double =   10 / 55    =    2  / 11

 

 

cool cool cool

 Oct 21, 2019

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