I did something similar when I was younger but not quite the same. 9+ anything is just an increment of one lower in that digit place with the resulting higher digit place carried to the next column. This when you see:
9+7, the 1’s column intuitively becomes a 6, carry the resulting 1 into the 10’s column and it’s 16. It works the same for any number where you intuitively know the relationship- 8+7 the 7 becomes a 5 instantly and the one carries to the 10 making 15. One can of course memorize basic math- but there is merit in applying a problem solving method to it as when math begins to become more complicated or unknown/not memorized having developed processes and understanding of the relationships between numbers can be helpful.
Honestly, I'm an aeromechanical engineer, and that's exactly how I do math in my head.
Instead of working with "weird" numbers, I usually bump them to the nearest 5 or 10, do the much easier math in my head, then give a quick thought to how far off, and in which direction I am.
It's not precise enough for production, but it's amazing at getting close estimate numbers during a meeting and impressing the management
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9+7, the 1’s column intuitively becomes a 6, carry the resulting 1 into the 10’s column and it’s 16. It works the same for any number where you intuitively know the relationship- 8+7 the 7 becomes a 5 instantly and the one carries to the 10 making 15. One can of course memorize basic math- but there is merit in applying a problem solving method to it as when math begins to become more complicated or unknown/not memorized having developed processes and understanding of the relationships between numbers can be helpful.
Instead of working with "weird" numbers, I usually bump them to the nearest 5 or 10, do the much easier math in my head, then give a quick thought to how far off, and in which direction I am.
It's not precise enough for production, but it's amazing at getting close estimate numbers during a meeting and impressing the management