What should I do with addition and subtraction in the first grade of primary school mathematics within twenty?
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Example 13+5.
13
+ 5
—————
18
Add numbers first. If it is greater than 10, then carry it, subtract 10 from the digits (for example, 5+8 digits are 13, and that digit becomes 3 or 10 digits), and add 1 to carry the digits.