Substitutes

I know that substitutes are necessary to keep the schools running, but what I don't understand is the lax requirements.  Case in point: our sub today wasn't sure how to evaluate -8 - (-4) because she hadn't done subtracting a negative number lately.  While I understand that is not something one does every day, she is a fully certified teacher for K-8, and she subs in preschool through high school.  How do we expect our 7th graders to learn the basics in math if their teachers aren't comfortable with the basics in math?  If I become a high school math teacher, I'm going to find a sub that I can trust - or else have a foolproof plan for when I get sick.  Egads.