Table seating question for real life use! I am trying to figure out how to rotate 75 people between 10 tables for 3 rotations with the people not overlapping when they rotate. Goal: for the attendess to meet new people at each table rotation. Thanks in advance!

For 81 guests
A neat way without requiring calculation is to arrange 81 names (or numbers) in a 9x9 grid. 
The tables for one arrangement are given by the rows.
The tables for the next arrangement are given by the columns.
The tables for the final arrangement are given by diagonals (wrapping around as if a tube has been formed by glueing together the top and bottom edges).
For 75 guests
Simply ignore 6 of the 'people'.
