Define $B$ to be a multi-subset of a set $A$ if every element of $B$ is an element of $A$ and elements of $B$ need not be distinct. The ordering of elements in $B$ is not important. For example, if $A = \{1,2,3,4,5\}$ and $B = \{1,1,3\}$, $B$ is a 3-element multi-subset of $A$. Also, multi-subset $\{1,1,3\}$ is the same as the multi-subset $\{1,3,1\}$.
How many 5-element multi-subsets of a 10-element set are possible? And Generalize your result to $m$-element multi-subsets of an $n$- element set $(m < n)$.
Adding it as an answer for closure. It should probably be marked as duplicate as it has an answer on the site, likely many times. Briefly, let $x_i$ be the number of times that $i$ is present. Then what you want is the number of solutions to $x_1+\ldots+x_n=m$ with $x_i \geq 0, x_i \in \mathbb{Z}$.