Digit Sums

The Digit sums are found by adding the digits in a number and then adding again if necessary. O and 9 are equivalent in this system.
37 = 10 = 1
461 = 11 = 2

Try these yourself:-
796
6437

Some tricks you can use are cast out 9s as adding 9s to a number does not change its digit sum. You can also forget numbers that add up to 9.
So 5317 (531) add to 9 so the digit sum is 7