There are 300 science and technology books in the library. The number of comic books is 100 less than 3 times of science and technology books, and the number of story books is 2 times of that of comic books. How many story books are there?

300 × 3 = 900
900-100 = 800
800 × 2 = 1600

There are 1800 story books in the school library, which is 4 / 5 of the number of comic books, and 5 / 7 of the number of literary books. How many literary books do the school library have

Comic books: 1 800 ／ 4 / 5 = 2 250 (copies); 2 250 ／ 5 / 7 = 3 150 (copies)

The number of literature and art books is four fifths of that of science and technology books. The number of science and technology books is only four fifths of that of comic books. There are 180 comic books. How many literature and art books are there

180×4/9×4/5=64

Given x (x-1) - (x2-y) = - 3, find x2 + Y2 / 2-xy
X2 is the square of X
Y2 is the square of Y
X2 + Y2 / 2 is two thirds of x2 + Y2

x²-x-x²+y=3
-x+y=3
x-y=-3
(x²+y²)/2-xy
=(x²-2xy+y²)/2
=(x-y)²/2
=(-3)²/2
=9/2

3、 Fourth, fifth grade trees, a total of 126 plants, how many trees each plant

If we plant x trees in the fourth grade, then we plant (X-13) trees in the third grade and (x + 22) trees in the fifth grade. We get (X-13) + X + (x + 22) = 126x-13 + X + 22 = 1263x = 126 + 13-223x = 117x = 39. So we plant 26 trees in the third grade and 61 trees in the fifth grade

Twenty eight out of twenty-nine out of seven of a number. What's two fifths of this number

145 out of 8

The ratio of the number a, B and C is 2:4:5, and the average number of the three numbers is 44?
What's the number C?

44＊3=132
132/(2+4+5)=12
12 * 2 = 24 (a)
12 * 4 = 48 (b)
12 * 5 = 60 (c)

Define the operation a * b = a, (AB), if 2 * 3 = 2, then the value range of x power of 1 * 2 is
The answer is (0,1]

This is to see 1 and 2 ^ x as a and B, respectively
When 1 ≤ 2 ^ x, 1 * 2 ^ x = 1, which is a constant
When 1 > 2 ^ x, 1 * 2 ^ x = 2 ^ x, but because 2 ^ x itself is greater than 0, 0

The sum of a and B is 16.5. If the decimal point of a is moved one place to the right, it is exactly equal to B. what are the numbers of a and B?
Set up the equation and solve it, we need the process

Let a be x, then B be 10x
x+10x=16.5
x=1.5
That is, the number a is 1.5 and the number B is 15

1. If the equation (A-1) x ^ 2 + X + A ^ 2-2a-1 = 0 about X is a quadratic equation, find the value of A
2. One change: if the root of the equation (A-1) x ^ 2 + X + A ^ 2-2a-1 = 0 is - 1, find the value of X
3. Bivariate: if one root of the quadratic equation (A-1) x ^ 2 + X + A ^ 2-2a-1 = 0 of X is 0, find the value of A

Ha, I'm slow

There are 300 science and technology books in the library. The number of comic books is 100 less than 3 times of science and technology books, and the number of story books is 2 times of that of comic books. How many story books are there?