There are the following activities to buy books: 1 ~ 9 books, 10% discount; more than 20 books (including 20 Books), 70% discount, each original price of 5 yuan, someone bought two books, a total of 30 books, cost 113 yuan, how many books each time (solve equations)

Let's buy x at one time and y at the other
x+y=30
5x0.9x+5x0.7y=113
So x = 8, y = 22

There are the following activities to buy books. For those who buy one to nine books, 10% off is allowed for each book. For those who buy more than 20 books (including 20 Books), 70% off is allowed for each book, and 5 yuan is allowed for each book
A total of 30 copies, a total cost of 129 yuan, ask how many copies to buy twice

Let's buy x for the first time and y for the second time
x+y=30
5×90％=4.5
5×70％=3.5
129÷30=4.3＞3.5
3.5x+4.5y=129
Find x = 24
y=6

There are the following activities to buy books: 10% off for 1-19 copies; 70% off for more than 20 copies (including 20 copies),
A classmate bought 30 books twice at a total cost of 129 yuan. How many books should he buy each time?
Quadratic equation with two variables

Suppose that the two books are x and y, then the system of equations can be divided into two cases
If you don't buy more than 20 books twice, then
x+y=30
5*x*0.9+5*x*0.9=129
There is no solution to the equations
Let x > 20
Then 5 * x * 0.7 + 5 * y * 0.9 = 129
x+y=30
The solution is x = 26, y = 4
Two books were bought for 26 and 4, regardless of the order before and after

In 2005, the price of a certain drug increased by 30% and decreased by 70% to 39 yuan in 2007. How much is the price of this drug before the price increase in 2005

Let the original price be x yuan
x（1+30%）（1-70%）=39
x=100
A: the original price is 100 yuan
complete!

It is known that the definition field of odd function y = FX is [- 3,3], and f (- 3) = 9. If FXZ is a decreasing function, then the minimum value of FX is!

If a (0, 2198), B (1, − 1, 58) and C (− 2, 1, 58) are three points in plane α, let a = (x, y, z) be the normal vector of plane α, then x: Y: Z=______ ．

AB = (1, − 3, − 74), AC = (− 2, − 1, − 74), α·ab = 0, α·ac = 0, x = 23yz = − 43y, X: Y: z = 23y: Y: (− 43y) = 2:3: (− 4). So the answer is 2:3: - 4

The price of a coat first increased by 10%, and then decreased by 10%. Now the price of this coat is ()% of the original price

It's 99% of the original price

Add + - to 123456789 to make it equal to 100

12+3+4+5-6-7+89=100

On the mathematical problems of ratio
The number ratio of red balls to white balls in the bag is 19:13. After several red balls are put in, the number ratio of red balls to white balls is 5:3. After several white balls are put in, the number ratio of red balls to white balls is 13:11. It is known that there are 80 less red balls than white balls in the bag, so how many balls are there in the bag?
No equations

A red ball is put into the bag for the first time, and the number of white balls is a + 80
When the ratio of red to white is 5:3, we can get (19n + a): 13N = 5:3
When the ratio of red to white is 13:11, we can get (19n + a): (13N + A + 80) = 13:11
The solution is n = 30, red ball 570, white ball 390

For a project, it takes 50 days to finish it alone and 60 days for Party B to do it alone. For two people to cooperate, Party A takes one day off for every three days and Party B takes one day off for every five days. How long does it take for Party A and Party B to complete the cooperation?

A does 1 / 50 every day, and a cycle of four days, the average daily completion of these four days is 1 / 50 × 3 △ 4 = 3 / 200;
And B does 1 / 60 every day, and six days a cycle, the average daily do 1 / 60 × 5 △ 6 = 1 / 72
Total number of days required 1 ÷ (3 / 200 + 1 / 72) = 450 / 13 (days)
It takes 450 / 13 days to complete the whole project

There are the following activities to buy books: 1 ~ 9 books, 10% discount; more than 20 books (including 20 Books), 70% discount, each original price of 5 yuan, someone bought two books, a total of 30 books, cost 113 yuan, how many books each time (solve equations)