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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.
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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)
the set
(b) of all different pairs (a,a), where "a" is integer positive number from 1 to 15 inclusive.
First set has = = 105 elements (individual dominoes).
Second set has 15 elements.
In all, there are 105 + 15 = = = dominoes.
ANSWER. 120 dominoes.
Solved.
Check it for double-seven set:
The formula + 7 gives = = 21 + 7 = 28 dominoes,
which is correct.