There are 1880 literature and art books, science and technology books and comic books in the library. 25 literature and art books have been lent, 50 science and technology books have been lent, and 40 comic books have been bought. At this time, the number of the three kinds of books is equal. How many of the original three kinds of books are there?

There are 1880 literature and art books, science and technology books and comic books in the library. 25 literature and art books have been lent, 50 science and technology books have been lent, and 40 comic books have been bought. At this time, the number of the three kinds of books is equal. How many of the original three kinds of books are there?

Let the number of three kinds of books be the same as X. according to the meaning of the title, we can get the following equation: X ÷ (1-25) + X + 50 + x-40 = 1880, & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; 53x + X + X + 10 = 1880, & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; & nbsp

There are 99 science and technology books, comic books and story books in the book corner of class Sany. The number of comic books is twice that of science and technology books, and the number of story books is three times that of comic books. How many of these three books are there?

Science and technology book = 99 (1 + 2 + 2 × 3) = 11
Comic book = 11 × 2 = 22
Story book = 11 × 2 × 3 = 66

2. A, B, C three cups each filled with some water, the amount of water in B cup is equal to the average amount of water in a, C two cups
There is some water in each cup. The amount of water in cup B is equal to the average amount of water in cup a and cup C. If 15 ml of water is added to cup C, the amount of water in cup a is equal to the average amount of water in cup B and cup C. which cup has more water than the other two? How many ML?

Suppose that the amount of water in the three cups is ABC
If the amount of water in cup B is equal to the average amount of water in cup a and cup C, then 2B = a + C > > C = 2b-a ①
If 15 ml water is added to cup C, then the amount of water in cup a is equal to the average amount of water in cup B and cup C, then 2A = (B + C + 15) ② > > C = 2a-b-15
Therefore, from ① to ②, 2b-a = 2a-b-15
2B-A-2A+B=-15
3B-3A=-15
B-A=-5
So a is bigger than B, cup a has more water than cup B, 5 ml more
I hope the answer will be useful to you

Let the sum of the elements in each row of the third-order matrix a be 3, and the vector α 1 = (- 12 - 1) ^ t α 2 = (0 - 11) ^ t be the solution of the homogeneous linear equation system AX = o
1. Write out all eigenvalues and corresponding eigenvectors of matrix A
2. Finding matrix A

1. The eigenvector corresponding to the eigenvalue 0 is α 1 = (- 1, 2 - 1) ^ t, α 2 = (0 - 1, 1) ^ t, because a α 1 = 0 * α 1, α 2 is the same. At the same time, the sum of elements in each row of matrix A is 3, so a (1, 1, 1) ^ t = 3 * (1, 1, 1) ^ t, another eigenvalue is 3, and the eigenvector is α 2 = (1, 1) ^

To produce a batch of parts, Party A can make 13 parts per hour, Party B can make 56 parts in 4 hours, and Party C can make one part in 12 hours. The working efficiency of three people is ()
A a is the highest, B B is the highest, C is the lowest

A can make 13 parts per hour
B can make 56 parts in 4 hours, 56 △ 4 = 14
It takes one twelfth of an hour for C to make a part
So choose B, C

(1) For a project, Party A works alone for 4 hours and Party B works alone for 6 hours. Party a works for 30 minutes first, and then Party A and Party B work together. How long can party A and Party B work together to complete the work
(2) When a youth team digs the chute, it gets 5 / 1 of the total length in the first day and 5 / 6 of the first day in the second day. If the digging speed continues at the speed of the second day, how many days can it be completed
(3) A ship arrived at wharf B downstream from wharf a, and then returned upstream to wharf C between wharf A and B. It has sailed for 7 hours. It is known that the ship has a range of 10 km at a still water speed of 7.5 km / h and a water flow speed of 2.5 km / h. The distance between wharf A and wharf B is calculated

1. Set a and B share x hours
1/4×0.5+（1/4+1/6）x=1
The solution is x = 1.5
2. Let's take X days (5 / 6 of the first day). Does that mean 1.2? If we don't change 6 / 5 to 5 / 6, it's OK
1/5+1/5×6/5(1+x)=1
The solution is x = 7 / 3
3. Let AB be the distance X
x/(7.5+2.5)+10/(7.5-2.5)=7
The solution is x = 50
Fractional representation (numerator / denominator)
Don't make a mistake in the future. It's easy to be misunderstood
O(∩_ ∩)O~

If 90 tons of grain are transported from a grain depot to B grain depot, then the two grain depots are equal, and a and B grain depots are equal
How many tons of grain were stored?

Total = 90 × 2 △ [7 / (7 + 5) - 5 / (7 + 5)] = 1080 tons
A = 1080 × 7 / (7 + 5) = 630 tons
B = 1080 × 5 / (7 + 5) = 450 tons
If you don't understand this problem, you can ask,

(x minus 9) multiplied by 1 / 12 = 8 to solve the equation

Yeah

1. It is known that the perimeter of an isosceles triangle is 63 cm. If an isosceles triangle is made with one waist as its side and its perimeter is 69 cm, then the base length of an isosceles triangle is ()
A 23cm B 17cm C 21cm D 6cm
2. If the sum of interior angles of a polygon is 1080 degrees, then the polygon is a () polygon
3. If the speed of a ship sailing downstream is 20 km / h and that of a ship sailing upstream is 12 km / h, then the speed of the ship in still water is (), and the water flow speed is ()
4. Given 2x-y = 3, then 1-4x + 2Y = ()
5. If a polygon is divided into 10 triangles by a diagonal line led by a vertex of the polygon, then the sum of the inner angles of the polygon is ()
6. Given x = 4t-1 / 2, y = t + 1 / 4, then the relation between X and Y is ()
7. The lengths of the three sides of a triangle are integers, and they are not equal to each other. The longest side of the triangle is no more than 5. The three sides of the triangle are ()
8. If 260 is added to a group of data at the same time, the new data has 24 numbers, the average is 43, and the mode is 35, then the original data has () numbers, the average is (), and the mode is ()
9. If data: x1, X2, X3, x4, meet the condition X1 < x2 < X3 < X4 < 0, then the median of data - x1, X2, - X3, X4 is ()
10. If the mode of the new data is 4, the median is 3, and the average is 5, then the mode of the original data is (), the median is (), and the average is ()

1、A
2、8
3、16、4
4、-5
5、360
6、y=x/4+3/8
7、3、4、5
8、24;-217;-225
9、1/2(x4-x3)
10、2004;2003;2005

It is known that {an} is an equal ratio sequence, Sn is the sum of its first n terms, A1 = 1, S3 = 7, and an ＞ 0. The first question is the general term formula for {an}
The second question: Let f (x) = x, and f (an) > F (an-1) + 4, find the range of n

Let the common ratio be q, a1 + A1 * q + A1 * Q * q = S3. The solution is q = 2 or - 3, because an > 0, so q = 2, the N-1 power of an = 2. According to the previous formula, the following inequality can be expressed as the n-th power of (1 / 2) * 2 > the n-th power of (1 / 4) * 2 + 4, because the n-th power of 2 > 0, so divide by the n-th power of 2 at the same time, We get the negative n-th power of (1 / 8) > 2, and the n-th power of F (x) = 2 is an increasing function, so the answer is n > 3,