I'm not entirely sure that's true. If you add 10+10, only the first number changes and you get 20. If you add 12+12, both digits change and you get 24.Also... That's so punny I almost dyed!
I'm lucky though, my brother is a math genius.
First Binary, then Hex. Octal can come later. Reasons: Binary is the language of computers. Hex is the math of programming. Octal is ... somewhere in between.