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Variations with repetition
In the football pools of $15$ matches, it is possible to mark the result of every match with $1$, $X$ or $2$. In how many ways is it possible to realize the football pools?
In this case, $n=3$ (because it is possible only to choose for every match either $1$, or $X$ or $2$), and $k = 15$ (because in total there are $15$ matches). Also, the order matters.
On the other hand, elements can be repeated (it is possible to mark more than one match with a $X$, for example). Therefore it is a question of varying the repetitions of $3$ elements from $15$ by $15$, that is to say: $$PR_{3,15}=3^{15}=14.348.907$$
There are $14.348.907$ possible football pools (which indicates that there is very little possibility of winning!)