Select nine numbers from the ten numbers of 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 to form a two digit number, a three digit number and a four digit number, so that the sum of the three numbers equals 2010, then the unselected number is______ ．

Select nine numbers from the ten numbers of 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 to form a two digit number, a three digit number and a four digit number, so that the sum of the three numbers equals 2010, then the unselected number is______ ．

The sum of all the digits in the addend and the sum of all the digits in the sum should be an integral multiple of 9. Because the sum of all the digits in 2010 is: 2 + 0 + 1 + 0 = 3, 0 + 1 + 2 + +9 = (1 + 9) △ 2 = 45; 45 is a multiple of 9, 3 plus 6 is a multiple of 9, so 6 should be removed from it

5 out of 21 to 3 out of 7

Multiply by 21
=5：9

4 and 3 / 8: 0.21

4 and 3 / 8: 0.21
=35/8:(21/100)
=35/8*100/21
=125:6

Three classes in grade one donated books to hope primary school. (1) class donated 152 books. (2) the number of books donated by class is the average of the three classes. (3) the number of books donated by class is
40% of the total number of grades. How many copies have the three classes donated?

152 (1-1 / 3-40%) = 570 volumes

Three classes of Grade 7 donated books to hope primary school. Class 1 donated 152 books. Class 2 donated the average number of books. Class 3 donated 40% of the total number of books donated by grade 3. How many books did the three classes donate?

Three classes donated x books. Class 2 donated X / 3. Class 3 donated 40% X
X-X/3-40%X=152
X* 4/11=152
X=418
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Class A, class B and class C donated books to hope project, among which one donated 6 books, two donated 7 books, and the rest donated 11 books; class B donated 6 books, three donated 8 books, and the rest donated 10 books; class C donated 4 books, six donated 7 books, and the rest donated 9 books. Class a donated 28 more books than class B, and class B donated 101 more books than class C. The total number of books donated by each class was between 400 and 500

Number of books donated by class a = 6 + 2 * 7 + (x-3) * 11 = 20 + 11 (x-3)
Class B = 6 + 3 * 8 + (y-4) * 10 = 30 + 10 (y-4)
Class C = 2 * 4 + 6 * 7 + (Z-8) * 9 = 50 + 9 (Z-8)
Class A - class B = 28
Class B - class C = 101
It can be seen from "the total number of books donated by each class is between 400 and 550",
Class a donated more than 529 books, class B donated more than 501 books
According to the number of books donated by class A is 20 + 11 (x-3), which is between 529 and 550, the number of books donated by class A is 537 or 548
If it is 537, then class B donates 509 books. We can see that the number of class B is not an integer, so we give up!
If it is 548, then class B donates 520 books and class C donates 419 books, which all meet the actual needs
The number of books donated by class A, class B and class C was 548, 520 and 419, respectively, with 51, 53 and 49

In order to celebrate a meaningful children's day, the Experimental Primary School launched an activity of donating books to a hope primary school
Help me understand it. Thank you. You'd better write the process

100÷5/6=120
（180-120）÷1.2=50…… V 1
50×1.2=60…… V 2
Equation:
There are x people in class 51 and 1.2x people in class 52; class 51 donates y books and class 52 donates y + 1
XY=100
1.2X(Y＋1)=180
The solution of the equations is x = 50

In order to celebrate a meaningful "June 1st" children's day, the Experimental Primary School launched an activity of donating books to a hope primary school. In the activity, class 1 of grade 5 donated 100 books, and class 2 of grade 5 donated 180 books. The number of students in class 2 was 1.2 times that of class 1. The average person in class 2 donated one more book than Class 1. How many students were there in each class?

Suppose there are x people in class one, then there are 1.2x people in class two. According to the meaning of the question: 100x + 1 = 1801.2x, the solution is: x = 50. The test shows that x = 50 is the solution of the original equation. 1.2x = 1.2 × 50 = 60. Answer: there are 50 people in class one and 60 people in class two

In order to celebrate a meaningful "June 1st" children's day, the Experimental Primary School launched an activity of donating books to a hope primary school. In the activity, class 1 of grade 5 donated 100 books, and class 2 of grade 5 donated 180 books. The number of students in class 2 was 1.2 times that of class 1. The average person in class 2 donated one more book than Class 1. How many students were there in each class?

Suppose there are x people in class one, then there are 1.2x people in class two. According to the meaning of the question: 100x + 1 = 1801.2x, the solution is: x = 50. The test shows that x = 50 is the solution of the original equation. 1.2x = 1.2 × 50 = 60. Answer: there are 50 people in class one and 60 people in class two

Three classes in grade seven of a school donated money to hope primary school. (1) the amount donated by the class is 1 / 3 of the total donation of grade seven
Then the donation amount of class (2) above is half of the sum of the donations of class (1) and class (3). It is known that class (3) has donated 380 yuan. How much is the total donation amount of Grade 7

Arithmetic method:
The number is "1"
(380 + 380 △ 2) △ 1-1 / 3-1 / 2 × 1 / 3) = 1140, which is the total amount,
1140 × 1 / 3 = 380 (1 shift)
(380 + 380) △ 2 = 380 (2 shifts)
Class 3 380