The best way to do this is try finding the combinations for all numbers starting in one...Originally Posted bykate

1,(2,3,4,5,6,7,8,9,10),(2,3,4,5,6,7,8,9,10)

The second digit can only have nine different numbers and the third can only have eight (because it can't have the same as the second or first one). Therefore the formula for all combinations starting with one is

now we multiply that by 10 to get all valid combinations for the first digit.