Addition: Appendix+Appendix = Sum.

1, recalculate the position where the addend can be exchanged;

2. Use and subtract one of the addends and the other addend that should be added to make it correct.

Subtraction: subtraction-subtraction = difference

1, the minuend minus the difference should be equal to the minuend;

2. subtraction+difference should be equal to the minuend.