if the question were
81 S in a S P
the answer would be
81 Squares in a Sudoku Puzzle
so the extra S letter is a mystery
unless
81 Squares in a Standard Sudoku Puzzle
as there are other variations
17 S in a H P
In a letter, "P. S." means "Post Script."
p = 9 (92 = 9 x 9 = 81)
81 squares on a sudoku grid
Assuming you mean the arithmetic mean, it is 80.
Post-script.
7 Sides on a Twenty Pence Coin
S-P interval means the integer minus the integer. The difference times nine.
P is a permutation. It is asking for the numbers in the ()'s after it. For exapmle, P(6,4) is 6x5x4 which is 120.
81 = square of nine
Five sides to a pentagon.
Standard and Poors.