What are the side effects if a person that has ADD takes a 20mg adderall?

It depends on how bad the person's ADD is. for some people, 20mg will make them focus to their full desire.

But keep in mind that adderall isn't successful for everyone- it is not always the right ADD medication for some people because of negative affects, such as extreme depression when you are coming down from it, irritability at unnecessary things, feeling a dependency to want it more often, etc.