Keep adding until you’re down to one digit. 4 + 7 = 11, 1 + 1 = 2.
“Casting out nines” is a trick I learned in about seventh grade. It’s a great way to check your arithmetic quickly. You add the digits together and keep adding them until you’re down to one, treating a 9 as zero. Then when you perform the computation, the answers should match. For example:
681 = 15 = 6 +
235 = 10 = 1
__________
916 = 7
If the actual total doesn’t reduce to a number that matches the 9less total, there’s a mistake in your arithmetic. It works for addition, subtraction, multiplication, and division.
So—using whatever answer you get when you’ve added the digits of your age together, and kept adding until you’re down to one digit, your age is going to be a multiple of 9 plus whatever your answer was.
If your answer is 2, then you’re 11, or 20, or 29, or 38, or 47, and so on. So…if I can guess your decade, I know your age.