30.65-(7.65+5.4)=30.65-7.65-5.4=23-5.4= 17.6

Simple mathematical calculation, what are the methods:

First, "rounding" is the first.

Two or more numbers that can be pieced together into a whole hundred are first combined for calculation, and additive commutative law, additive combination law and other methods are used to facilitate calculation.

Second, change the operation order: in the mixed formula with only "+"and "-",you can change the operation order.

One trick of this type is that no matter which number is moved, the previous serial number will also be "moved".

For example, 36-5+4 can be changed into 36+4-5 by moving, the positions of 4 and 5 have changed, and the serial numbers before 4 and 5 have also changed.

Third, calculate the sum of arithmetic continuous numbers.

A sequence with equal difference between two adjacent numbers is called arithmetic connected number, also called arithmetic progression. If the first number is a, the last number is b, and the difference between two adjacent numbers is c, then the law of this series can be f=a+(n- 1)c, where n is the total number of this series, then the sum d of this series can be expressed by the formula: d=n(a+f)/2.

Fourth, the quasi-radix method.

This method is to make every number in the formula equal to an integer first, and then solve it according to the method of "more is less".

Mathematics, as one of the three main courses in primary schools, plays an important role in any stage of the examination. The operations of mathematics in primary schools mainly include addition, subtraction, multiplication and division and simple multiplication.

All the more difficult calculations are inseparable from the evolution of these basic operations. Many parents left a message to Xiaohui saying that children can't do simple calculations and math is difficult to learn. In fact, as long as they master some laws and remember some formulas, the calculation will become simple.

In the process of calculation, we should use different methods to improve the skills of calculation methods, cultivate the thinking of mathematical calculation, and pay attention to the good character of carefulness and patience.