Exponential GF application

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I have $15$ different books I have $5$ child. I want to give it all to all my child where every my child get at least $1$ book How many way I can distribute it????

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What you want is to count the number of surjective functions from a set of size $15$ to a set of size $5$. The number of ways to do this is ${15 \brace 5}\times 5!$. where $15 \brace 5$ is the Stirling coefficient of the second kind.

Why? every surjection induces a partition of the domain (the set of books that go to child $1$, the set of books that go to child $2$ etc. So once you select a partition of the domain you just have to select which block of books goes to which child in $5!$ ways.