A permutation is an arrangement in which order is important. The notation for permutations is P(n,r) which is the number of permutations of "n" things if only "r" are selected.

Example: If nine students take a test and all have different scores, the highest score could be achieved by any of the nine students. The second highest score could be achieved by one of the remaining 8 students. The third highest score could be achieved by one of the 7 remaining students.

The number of possible permutations would be: P(9,3) = 9*8*7 = 504 possible arrangements of the top three scores.