First of all, it depends on whether you have learned rounding.
The situation of adding and subtracting first generally appears before learning the rounding method, and the topic is likely to be "estimation, xxxx", without specifying which digit is accurate.
When calculating this kind of questions, if you add or subtract, it is more convenient to treat the two numbers as integer hundred, integer thousand or integer ten, and then add or subtract the approximate numbers to get the final answer.
In the case of multiplication and division, the number with more digits is regarded as an integer hundred, an integer thousand or an integer ten, and then the final answer is obtained by multiplication and division.
This kind of topic mainly examines students' estimation or quick calculation methods, helps students improve their sense of numbers, and exercises this kind of topic can improve students' ability to apply mathematics in life.
The situation of addition, subtraction and further subtraction appeared after the rounding method was systematically studied. By this time, students should have learned decimal places. The topic will generally explain "accurate to xxx" or "keep X decimal places". This is the rounding method that we contact and use most often.
Extended data:
Rounding:
Rounding is an accurate counting reservation method, which is essentially the same as other methods. But the special point is that the difference between the reserved part and the actual value cannot exceed half of the last order of magnitude: if there is a probability of 0 ~ 9, the error sum of this method is the smallest for a large number of reserved data.
This is why we use this method as the basic booking method.
Example: For example, π is rounded, leaving 3. 14. But sometimes you can use "one-in method" and "one-out method" instead of rounding. The rounded fourth house is 1, 2, 3, 4 and the fifth house is: 5, 6, 7, 8, 9.
For example, 288 students have a spring outing, and 45 people have a bus, which is 6.4 buses. However, you must get into a bus to avoid letting more people out and fewer cars. Because the number of cars cannot be decimal, seven buses are needed.
For another example, 10 16 liters of gasoline, if you want to refuel a car, you can add 50.8 cars on average to 20 liters, but you must remove the tail to let more cars come out and less fuel. Because the number of cars cannot be decimal, only 50 cars can be refueled.
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