There is nothing wrong with the answer, but the second multiplied by 5 equals the first, that is, the first equals the second multiplied by 5, and the two molecules in this step are 9 respectively! /4! ,9! /5! This is the first 9! /4! =5*9! /(5*4! )=5*9! /5! , is the second 9! /5! ? 5 times (that is, the second 9! /5! ? (9-5+ 1) times)

Correlation conversion formula of permutation number: