I expect my girlfriend to be able to do simple math.
Why do you have that expectation about your girlfriend?
She's been very bad at math for a long time and I'm tired of dealing with the constant problems that result from her deficiency with numbers.
While can be frustrating to be with a partner who has issues with dyscalculia, you should offer patience and support to your partner so that she can overcome that issue. Partners should help one another improve and helping your partner learn simple math is something that you should do for the benefit of both of you.
Are you saying I can't ask something simple of my girlfriend. She's 24 years old and should know how to add in her head.
Not everyone approaches numerical problems with the same degree of proficiency. Some are stronger verbally; others are more mathematically inclined. One of the advantages of a relationship is the unique aptitudes and spheres of knowledge each person contributes -- it's case of the whole being greater than the sum of its parts.