This is not necessarily an indication of ADHD. Being said, neurodivergence could lead people to attempting new solutions to tasks that they have never been taught, so it's not like there's no correlation. There's just no direct causation here.

Being said, this is a fantastic mental math shortcut, one that I wish would get taught more. Works for adding, subtracting, hell even multiplying and dividing. 16 × 20 is 20 more than 15 × 20, which is 300 + 20 = 320. Or perhaps you could say that 16 × 20 is 16 × 10 × 2 = 160 × 2 = 320. There are so many ways to break large numbers down into smaller, more manageable numbers for arithmetic, makes mental math a hell of a lot easier. And you can impress people with the ability to multiply large numbers in your head if you get good at it.

57 × 41 is 57 more than 57 × 40, which is (7×40) more than 50×40. Well, 50×40 is 2,000, so 57×41 = 2,000 + (7×40) + 57 = 2,000 + 280 + 57 = 2,280 + 57 = 2,337. Admittedly that took me a minute and I did originally get 2,327, but you can see the process.