Percentages are incommensurable with most things, including other percentages in the general case. They are percentages _of something_ and in general the only way you can do addition, subtraction, or comparison is to identify the referent and multiply it out first (the special case is when the other value involved is another percentage with the same referent). So in math class, when you see 20%, you translate it to ".2 _times something_", which is not a value that can be added to 10. You have to figure out what to multiply it by first. In the case of 10+20%, it would be reasonable to assume 20% of 10, which is how you get 12. It would also be reasonable to ask "20% of what?" 10.2 is 10 + 20% of 1, which requires an explanation of how that 1 got involved.

If you oveload "+" to mean whatever-you-want, it's possible. Using bad python [1], it's something like:

  def sum(x,y):
    if is_number(x) and is_number(y):
      return x + y
    elif is_number(x) and is_percent(y):
      return x * (1 + y.value/100)
    else:
      raise fatal_error

I don't claim it's a good idea, but it's possible if the language allows operator overload.

[1] Sorry, my main current language is Racket, but I thought that python-like is easier to read for most people, in spite people that likes python can find like 10 error in 7 LOC.