lol, it is pretty bizarre I know. I just know 9 breaks into 3+3+3 because it’s a square number, and adding one of those 3s to 7 makes it 10, which is easier to add stuff to, then I just get rid of the remaining 3s by adding them to 6, then 10+6 is a very easy equation to intuitively add, because you just replace the “0” with “6” to get “16” and you’re done.