In the first volume of the third grade, add and subtract hundreds and dozens of crossings to calculate problems. Such a problem is very simple to calculate. When calculating, you can add and subtract whole hundred first, then add and subtract whole ten, and finally add and subtract single digits, which is easier to calculate.

For example:

270+560=830

620+ 180=800

960+ 1 10= 1070

Law of addition:

When using "truncation method" in addition and subtraction, add and subtract directly from the upper left bit (and pay attention to whether the next bit needs carry and dislocation) until you get the answer with required accuracy. When using truncation method in multiplication or division, in order to make the result as accurate as possible, we need to pay attention to the direction of truncation approximation:

First of all, to expand (or shrink) a multiplier, we must shrink (or expand) another multiplier.

Second, to expand (or reduce) the dividend, you need to expand (or reduce) the divisor. If it is the sum or difference of two products (that is, a*b+/-c*d).

Third, expand (or shrink) one side of the plus sign. You need to shrink (or expand) the other side of the plus sign.

Fourth, to expand (or shrink) one side of the minus sign, you need to expand (or shrink) the other side of the minus sign.