A bookshelf has two layers of books, and the books on the upper layer account for 40% of the total. If 48 books are taken from the upper layer and put into the lower layer, then the books on the lower layer account for 75% of the total. How many books are there in this bookshelf?

A bookshelf has two layers of books, and the books on the upper layer account for 40% of the total. If 48 books are taken from the upper layer and put into the lower layer, then the books on the lower layer account for 75% of the total. How many books are there in this bookshelf?

Suppose there are x books in total, we can get: 40% x-48 = (1-75%) x 40% x-48 = 25% X & nbsp; & nbsp; 15% x = 48 & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; X = 320 A: there are 320 books in this bookshelf

A bookshelf has two layers of books, and the books on the upper layer account for 40% of the total. If 48 books are taken from the upper layer and put into the lower layer, then the books on the lower layer account for 75% of the total. How many books are there in this bookshelf?

Suppose there are x books in total, we can get: 40% x-48 = (1-75%) x 40% x-48 = 25% X & nbsp; & nbsp; 15% x = 48 & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; X = 320 A: there are 320 books in this bookshelf

It is known that the parabola y = 1 / 2x2 + BX + C and X-axis intersect at two points AB and y-axis intersects at point C through two points BC is y = 1 / 2x-2 connecting AC if it is in triangle

Because B and C are on the straight line y = 1 / 2x-2, B (4,0) and C (0, - 2) get b = - 3 / 2, C = - 2 (Note: simple substitution evaluation is not more said), so a (- 1,0), B (4,0), C (0, - 2) get the straight line AC: - 2x-y-2 = 0, and the straight line BC: x-2y-2 = 0, because △ ABC is a right triangle, and the angle c is equal to 90 degrees

If the molar mass of a substance is m, the density ρ, the Avogadro constant n, then what is the number of molecules in the substance with volume V?
One is n ρ V / m, the other is n / m or M / N?

NM

Define a kind of operation "*", for positive integer n satisfying the following operation properties: (1) 1 * 1 = 1, (2) (n + 1) * 1 = 3 (n * 1), then n * 1 is represented by an algebraic expression containing n______ ．

According to the meaning of the title, 1 * 1 = 1 = 30, then we can get: 2 * 1 = 3 (1 * 1) = 3 = 31, 3 * 1 = 3 (2 * 1) = 3 × 3 = 9 = 32 n*1=3[（n-1）*1]=… =The answer is n * 1 = 3n-1

The resistance of a small bulb is 10 Ω, and the normal working voltage is 2.5V. (1) can you connect the small bulb directly to 3V power supply?
The resistance of a small bulb is 10 Ω, and the normal working voltage is 2.5V
(1) Can you connect this small bulb directly to the 3V power supply?
(2) If not, how to connect a group value to make the bulb light normally?

When the bulb is working, the current is I = 2.5/10 = 0.25 (a). If you want to drop 0.5 V under the condition of 0.25 a, then
R = 0.5 / 0.25 = 2 (Ω) in series with a 2 ohm resistor

Help to calculate √ 5 & # 178; - 4 & # 178; × 4O × (2 & # 179; - # 64) thank you

√5²-4²×4o×（2³-√64）
=5-16*40*（8-8）
=5-0
=5

Simple and convenient calculation in primary school
24 / 29 - (4 / 25-4 / 9) simple calculation
Wrong input
29/24-（4/25-4/9）

What's 24 29 - (5 / 24-4 / 9)
=(29-5 / 24) + 4 / 9
=1 + 4 / 9
=1 and 4 / 9
29/24-（5/24-4/9)=29/24-5/24+4/9=1+4/9=13/9

If there is only one point in common with hyperbola x24 − Y23 = 1, then the number of lines L is______ ．

Let the straight line L: y = KX + 3, and substitute it into hyperbolic equation to simplify it to (3-4k2) x2-24kx-48 = 0. ① to make L and hyperbola have only one common point, only one or two real roots of the above equation should be equal, ｜ 3-4k2 = 0, or △ = 192 (3k2 + 3-4k2) = 0, and then K2 = 34 or 3, ｜ k = ± 32 or ± 3, so there are 4 answers to the problem

How much resistance of 220 V voltage string can make 3 V LED light on

What is 3V led