# Distributing n balls into k boxes so that every box has an even number of balls

How many ways can you distribute n balls into k boxes so that every box has an even number of balls?

Well, so far I think than n must be an even number. And I know that it basically starts with a simple equation:

$$x_1 + x_2 + x_3 + \cdots + x_k = n$$ where $$x_i\equiv 0\pmod 2$$ Normally I would use a combination with repetition but since there has to be an even number of balls in every box I don't know what to do.

Of course $$n$$ must be even. Then distribute $$\frac{n}{2}$$ balls over $$k$$ boxes (no conditions) and double the amounts in all boxes. This gives all such even distibutions and so the problem is equivalent to the $$\frac{n}{2}$$ over $$k$$ boxes problem for even $$n$$. For $$n$$ odd there are no solutions.
Hint: Let $$n = 2m$$. Let $$x_i = 2y_i$$, $$1 \leq i \leq k$$. Then \begin{align*} x_1 + x_2 + x_3 + \cdots + x_k & = n\\ 2y_1 + 2y_2 + 2y_3 + \cdots + 2y_k & = 2m\\ y_1 + y_2 + y_3 + \cdots + y_k & = m \end{align*} which is an equation in the nonnegative integers.