I get the correct results at all the tests when the number of balls
is less than 15. Any test over that is plain smaller. I'm using the
following sequence to calculate the result:

x1 = 1 + nb
xn = xn-1 * ( nb + n ) * ( 1 / n )

where nb is the number of boxes. I use this function for both
colours and then multiply the two results.

Is there something wrong with it?