Originally posted by c0mplex:
total number of ways = 9!/(4!4!) = 630
treating RB as 1 'ball', number of ways = 8!/(4!3!)x2 = 560
required no. of ways = 630 - 560 = 70
your method is logical too... hmm, different answer...
you cant treat RB as 1 'ball' and permute, this will result in
lots of overlap. (that's why your answer of 560 is so large)
lets see why:
one of your perms: W (R B) R W W R R (where R B is a group)
another of your perms: W R (B R) W W R R (where B R is a
group)
err...arent these 2 ways the same? 
