Explanation:
The single digits are {0,1,2,3,4,5,6,7,8,9}
So naturally 0 is the first thing to test.
The number 270 is a multiple of 9 because 9*30 = 270. The digits of 270 add to 2+7+0 = 9 showing that it's a multiple of 9. Refer to the "divisibility by 9 rule".
The next possible value of y will be 9 units higher than y = 0, so we go for y = 9. Meaning that 279 is also a multiple of 9 (because 2+7+9 = 18 is a multiple of 9).
