I think we can assume every b is a box and every a is a ball, and it looks like there are 10 boxes and 6 balls. So I think there are C(15 5) (15 choose 5) ways for the combination. But the correct answer is 11 choose 5. I wonder why.
The original question is 'How many ways of arranging 6 a's and 10 b's with no consecutive a's?'