In how many ways can 9 examination papers be arranged so that the best and worst never come together

See, the question, there have been give that best and worst papers never come together. In the second step of the solution, best and worst paper have been taken together means, best + worst = One new type of paper. And the total examination paper is 10 and when best and worst paper taken together it become (10 -1) i.e. 9. And these 9 papers can be arranged themselves into 9! ways.