1, two-digit plus two-digit carry addition: formula: add 9 minus 1, add 8 minus 2, add 7 minus 3, add 6 minus 4, add 5 minus 5, add 4 minus 6, add 3 minus 7, add 2 minus 8 plus/kloc-0 minus 9.

2, two-digit subtraction, two-digit abdication subtraction. Oral calculation: formula: minus 9 plus 1, minus 8 plus 2, minus 7 plus 3, minus 6 plus 4, minus 5 plus 5, minus 4 plus 6, minus 3 plus 7, minus 2 plus 8, minus 1 plus 9.