Cardinality of Solutions to an Inequality

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Show that the number of solutions in nonneg. int. of the ineq. $$x_1+x_2+\cdots +x_n\leq M,$$ where $M$ is a nonneg. int., is $C(M+n,n)$.

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HINT: The number of solutions in non-negative integers to

$$x_1+x_2+\ldots+x_n\le M$$

is the same as the number of solutions in non-negative integers to

$$x_1+x_2+\ldots+x_n+x_{n+1}=M\;,$$

which is a standard stars-and-bars problem.