First of all, I know that multiplication is a simple addition formula and division is a simple subtraction formula.

For example, if you add up four consecutive ones, you can count how many fours there are. Suppose six fours can be calculated by 6×4, which is much simpler than 4+4+4+4+4+4.

Then recite the multiplication formula table, skillfully calculate one digit multiplied by one digit, skillfully calculate two digits multiplied by one digit, two digits multiplied by two digits, three digits multiplied by one digit, three digits multiplied by two digits, three digits multiplied by three digits ... You can basically multiply with any number in the future.

Division is the same, first one digit is divided by one digit, two digits are divided by one digit, and two digits are divided by two digits. ...

Mathematics is the same.