1. conversion rule. The estimator transforms the mathematical structure or equation into a form more convenient for mental operation or a realistic situation close to life experience through the transformation process. Example: 78655+86421+96509+93421+106409 =, the estimator may convert the problem into multiplication form based on "number sense". 90000 times 5 is 450000(90000×5=450000), and the answer should be almost 450000. 2. Reorganization does not change the mathematical structure of the topic, but changes the number into a form that is easier to calculate in mind. For example, to calculate 26 162÷254, 26 162 can be regarded as 25000, 25000 ÷ 250 = 100, so 26 162÷254. If the calculation result is 13, it must be wrong. 3. Compensation method This is to identify and compensate inaccurate answers obtained through the process of "transformation" or "reorganization". The outstanding feature of this thinking process is that the operation is relatively accurate. For example, when teaching fractional multiplication 29.6×3.8, students often make calculation errors due to calculation or decimal point position movement. In view of this situation, students can be skillfully guided to estimate: 29.6≈30, 3.8≈4, so 29.6×3.8≈ 120, less than 120. The factor should also be 8 in the end. In this way, if students find the result in the calculation (such as 1 12.46 or 1 124.8). Then the result must be wrong.