n - number of white balls
2n - number of black balls
In how many ways you can put it into n boxes? It have to be at least one black ball in each box.
My idea: First of all let's put one black ball in each box. We have got n white balls and n black balls to put into n boxes so my problem is how to put 2n balls into n boxes. So it's like solving an equation: $x_1+x_2+...+x_n=2n (x_n>=0)$ Is it correct?