Analysis: this question is a typical inclusion problem, and the law of solution is: both a and b =A+B- total (in two cases).
According to "there are 28 people who answered one question correctly and 32 people who answered two questions correctly", their total number is 28+32=60, and the number of people who answered two questions correctly is calculated again. Then, according to the principle of inclusion and exclusion, the number of people who answered two questions correctly is 60-42= 18 (person), and the answer can be made accordingly.
The problem solving process is as follows:
Solution:
28+32-42
=60-42
= 18 (person)
A: Some 18 people answered these two questions correctly.
Algorithm:
Addition and subtraction algorithm
Integer:
1, aligned with the same number.
2, from the unit.
3. When the addition reaches dozens, it will be promoted to a higher position; If the subtraction is not enough, subtract 1 from the high position. When this number is added to 10, it will decrease.
Nature of operation:
Properties of addition operation
From additive commutative law's associative law, it can be concluded that when several addends are added, the positions of addends can be exchanged at will; Or add a few addends first and then add them with other addends, and the sum remains the same. For example: 34+72+66+28 = (34+66)+(72+28) = 200.
Subtraction operation property
1, a number MINUS the sum of two numbers is equal to subtracting each addend in the sum from this number in turn. For example:134-(34+63) =134-34-63 = 37.
2. A number minus the difference between two numbers is equal to this number minus the minuend in the difference first, and then minus it. For example: 100-(32-15) =100-32+15 = 68+15 = 83.
Subtract a number from the sum of several numbers. You can choose any addend to subtract this number and then add it to the rest of the addends. For example: (35+17+29)-25 = 35-25+17+29 = 56.
4. If a number subtracts several numbers in succession, you can add all the subtractions first, and then subtract the sum of the subtractions from the minuend. For example: 276-115-85 = 276-(115+85) = 76.