SOLUTION: How many dominoes are in a double-fifteen set? Pictured below are double-three and double-four sets. Remember that there will be double-zero, double-one, and so on to double fiftee

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Question 1196608: How many dominoes are in a double-fifteen set? Pictured below are double-three and double-four sets. Remember that there will be double-zero, double-one, and so on to double fifteen, and that one-zero is the same piece as zero-one.
Answer by ikleyn(52876) About Me  (Show Source):
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How many dominoes are in a double-fifteen set? Pictured below are double-three and double-four sets.
Remember that there will be double-zero, double-one, and so on to double fifteen, and that one-zero
is the same piece as zero-one.
~~~~~~~~~~~~~~~

This set is the set 

    (a)  of all different ordered pairs (a,b), where "a" and "b" are positive integer numbers 

         1 <= a < b <= 15   (where "a" and "b" are different)


highlight%28highlight%28PLUS%29%29  the set 

    (b)  of all different pairs (a,a), where "a" is integer positive number from 1 to 15 inclusive.


First set has  C%5B15%5D%5E2 = %2815%2A14%29%2F2 = 105 elements (individual dominoes).

Second set has 15 elements.


In all, there are 105 + 15 = highlight%28highlight%28120%29%29 = %2815%2A%2815%2B1%29%29%2F2 = %2815%2A16%29%2F2 dominoes.


ANSWER.  120 dominoes.

Solved.


Check it for double-seven set:

The formula  C%5B7%5D%5E2 + 7  gives  %287%2A6%29%2F2+%2B+7 = 42%2F2+%2B+7 = 21 + 7 = 28 dominoes,

which is correct.