There are (n+1) white and (n+1) black balls each set numbered 1 to (n+1). The number of ways in which the balls can be arranged in a row so they the adjacent balls are of different colours is ?

1 Answer
Jun 29, 2018

2*[(n+1)!]^2

Explanation:

Visually, there are two situations in which the colours will wind up alternating. These will appear as either:
BWBWBW....
or
WBWBWB....
That's where the factor of 2 comes from.

We multiply 2 by the number of ways of rearranging the n+1 white balls. That is (n + 1)!. We also multiply it by the number of ways of rearranging the black balls. Another (n + 1)!. The answer is
2[(n+1)!]^2.