The definition of finding positive and negative numbers

The definition of finding positive and negative numbers

The distance from the point on the positive number axis to the origin
The opposite number of the distance from the point on the negative number axis to the origin
The positive number is greater than 0 and the negative number is less than 0

1. 60% of a number is 1.2 less than 1.8
2. What is 40% of a number equal to one sixth of 48
3. What is the result of 20% reduction and 20% increase

Solution 1: (1.8-1.2) △ 60 ℅ = 1, this number is 1
Solution 2: 48 × 1 / 6 △ 40 ℅ = 20
Solution 3: 100 × (1-20 ‰) × (1 + 20 ‰) = 96,

The shop is offering a discount. Xiaoming bought two books, SanZiJing and 100000 whys. The discount is as follows: the original price of SanZiJing is 8.50 yuan. The original price of 100000 whys is 11.50 yuan, 20% off. How much yuan can Xiaoming save?

It's complicated for you to understand
8.50 * 85% = 7.225 yuan
11.5 * 85% = 9.775 yuan
8.50-7.225 = 1.275 yuan
11.5-9.775 = 1.725 yuan
1.275 + 1.725 = 3 (yuan)
It's simple
(8.5 + 11.5) * (1-85%) = 3 (yuan)

A simple algorithm for solving mathematical formula
（1）888×667+3333×6666 （2）1234×4326+2468×2837
（3） 9999×1222－3333×6666
Attention is a simple algorithm! Good plus 100 points
The first problem should be simple algorithm, no more than 6 steps! fast

888×667+3333×6666
=888×666+888×1+3333×666+3333×6000
=888×666+888×1+333×666+3000×666+3000×6000+333×6000
=111×111×(8×6+3×6)+111×(8×1+18000+18000)+18000000
=111×111×66+111×36008+18000000
=111×(7326+36008)+18000000
=111×43334+18000000
=22810074
（2）1234×4326+2468×2837
=1234 * 4326+1234 * 5674
=1234*（4326+5674）
=1234*10000
=12340000
（3）9999×1222－3333×6666
=9999 * 1222- 9999*2222
=9999*（1222-2222）
=9999*（-1000）
=-9999000

1) Car a and B drive from a to B. car a runs 60km per hour. Car B runs 15km more per hour than car A. car B starts two hours after car a leaves. As a result, two cars arrive at B at the same time. Find the distance between a and B
2) My father is 28 years older than my son this year. Four years ago, my father was 4.5 times older than my son. How old are my father and my son this year?

1) 60 + 15 = 75 (km) 60 * 2 = 120 (km) 120 / 15 = 8 (hours) 8 * 75 = 600 (km)
A: A and B are 600 km apart
2) The age of father is 4.5 times that of son, and the age of father is 3.5 times that of son
Four years ago, my son was 8 years old. Now my son is 8 + 4 = 12, and my father is 12 + 28 = 40
A: my father is 40 years old and my son is 12 years old

It's a formula
There are some apples and peaches in Mingming's family. The number of apples is three times that of peaches. Mingming and his father eat one apple every day, and his mother one peach every day. After a few days, there are still 9 apples and 0 peaches left. How many apples do you have

There are 27 apples and 9 peaches. Suppose there are x apples and Y peaches, because the number of apples is three times that of peaches, so x = 3 * y. after n days, there are still 9 apples and 0 peaches left. Because Mingming and his father eat one apple every day, and his mother eat one peach every day, so they consume 2 apples every day, so they consume 2 * n after n days

A passenger ship sails downstream from port a to port B at a speed of 38 km / h, and then sails upstream from port B to port a for 8 hours. Given that the water speed is 8 km / h, how many km is the distance between port a and port B?

The speed of the ship in still water is 38-8 = 30 km / h
The speed of the ship in the upstream is 30-8 = 22 km / h
The distance between the two places is 22x8 = 176km

What's the formula
Xiao Pang and Xiao Ding use a rope to measure an ancient tree. They use the rope to circle four times, with a difference of 0.55 meters. As a result, they circle three times, but with an increase of 1.65 meters. What is the perimeter of the ancient tree?

Let its circumference be X. The equation is 4x-0.55 = 3x + 1.65, and the solution is x = 2.2

On the national day, a total of 52 vehicles were inspected. Each vehicle was 4 meters long, and the distance between two adjacent vehicles was 6 meters. The motorcade drove 105 meters per minute. The motorcade had to pass the 536 meter long inspection site, and it had to pass the inspection site______ Minutes

The length of the motorcade is: 52 × 4 + (52-1) × 6, = 208 + 306, = 514 (m); the time required for the motorcade to pass the inspection site is: (514 + 536) △ 105, = 1050 △ 105, = 10 (min). A: it takes 10 minutes for the motorcade to pass the 536 m inspection site

A physics problem. I can't understand the last step. How can I get the formula?
In order to determine the sediment content in the 1998 flood of the Yangtze River, that is, the mass of sediment in each cubic decimeter of flood, the researchers collected 40 cubic decimeter of water sample, and measured its total mass as 40.56 kg. The known dry sediment density is ρ mud = 2.4G / cm & # 179;, trying to find out the sediment content in the flood?
ρ flood = m flood / V flood = 40.56 kg / 40DM & # 179; = 1.014 × 10 & # 179; kg / M & # 179;,
M flood = m water + m mud = ρ water V water + ρ mud V mud,
The density of flood is: ρ flood = m flood / V flood = (m water + m mud) / V flood = ρ water V water / V flood + m mud / V flood,
V water = V flood - V mud = V flood - M mud / ρ mud,
Ψ m mud / V flood = (ρ flood - ρ water) ρ mud / ρ mud - ρ water = (1.014-1) × 10 & # 179; × 2.4 × 10 & # 179; / 1.4 × 10 & # 179; / / kg / M & # 179; = 24 kg / M & # 179
I don't know how I got the last step?

ρ Hong = m Hong / v Hong = (m water + m mud) / v Hong = ρ water V water / v Hong + m mud / v Hong, extracting m mud / v Hong, substituting m mud / v Hong = ρ Hong - ρ water V water / v Hong into v water = v Hong - V mud = v Hong - M mud / ρ mud, obtaining m mud / v Hong = ρ Hong - ρ water (v Hong - M mud / ρ mud) / v Hong = (ρ Hong - ρ water) + water m mud / v Hong ρ mud, simplifying the above formula: M