I used to be terrible at it! subtracting and ect. was a simple formula, the fact that the number always came up wrong was the problem! Every month DH would have to go find my math error or help me find the missing bill, or oddly enough an extra one I put in, or forgot to check off.
I now can do it alone, but I have been with him for 7 years
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Best thing he did, was sit there and let me talk outloud till I got stuck, then he would go find the problem. Instead of just doing it for me, and when he had to find the problem he would show it to me and I would redo the math, so I learned.