One of the worst things that you can do for someone who is having trouble with math is get frustrated. Because then they get frustrated with themselves, and that frustration makes them worse at math, completing the destructive cycle.

Therefore if you get frustrated dealing with her learning trouble, you're contributing to her problems. You need someone who can deal with her issues, deal with her frustration, and can break the problem down even farther until they find what steps she's missing.

Cheers,
Ben