On a machine that pays 31-for-1, the machine keeps your bet when you make it, but pays 31 units on a win still a total of 31 on your side after a win. So 31-for-1 is the same as 30-to-1, 16-for-1 is the same as 15-to-1, 10-for-1 is the same as 9-to-1, and so on.