100+50-30= 120
70-20+40=90
90-60+30=60
40+20- 10=50
80-50+30=60
60+ 10-40=30
30+70-50=50
90-40+20=70
50+30-20=60
80-60+ 10=30
Operation attribute
1, the 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.
2. The nature of subtraction operation
(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 the minuend. For example: 100-(32-15) =100-32+15 = 68+15 = 83.
(3) Subtract a number from the sum of several numbers. You can choose any addend to subtract this number, and then add it with the rest addends. For example: (35+17+29)-25 = 35-25+17+29 = 56.
(4) 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.
3. The nature of multiplication operation
(1) Multiply several numbers by the product of a number, so that any factor in the product can be multiplied by this number and then multiplied by other numbers. For example: (25× 3× 9 )× 4 = 25× 4× 3× 9 = 2700.
(2) Multiply the difference between two numbers by a number, so that the minuend and the minuend are multiplied by this number respectively, and then subtract the products. For example: (137-125) × 8 =137× 8-125× 8 = 96.
4. The nature of the division operation
(1) If a number is divided by (or multiplied by) a number and then multiplied by (or divided by) the same number, the number remains the same. For example: 68 ÷ 17× 17 = 68 (or 68× 17 ÷ 17 = 68).
(2) A number divided by the product of several numbers can be divided by the factors in the product in turn. For example: 320÷ (2× 5× 8) = 320÷ 2÷ 5÷ 8 = 4.