Please use the balance, empty bottle and water to design an experiment to measure the density of milk. Write out the physical quantity to be measured and deduce the formula for calculating the density of milk

Please use the balance, empty bottle and water to design an experiment to measure the density of milk. Write out the physical quantity to be measured and deduce the formula for calculating the density of milk

First measure the mass a of the empty bottle, then fill the empty bottle with water, pay attention to make a mark after filling the water, then measure the mass B of water + bottle, and then B-A = the mass of water
Then pour the water out of the bottle and pour the milk into the place marked before. This is to ensure the same volume of water and milk. Measure the mass C of milk + bottle, and then C-A = the mass of milk
Since the volumes of water and milk are equal, the density of milk / the density of water = (the mass of milk / the volume of milk) / (the mass of water / the volume of water) = the mass of milk / the mass of water = (C-A) / (B-A), the density of water = 1
Then we get the formula of milk density

Three teachers led 72 students on a spring outing. Each of them took a bottle of drinks. The first one was 13 yuan, 8 bottles in a box. The second one was 30 yuan, 20 bottles in a box. Which kind of drinks saved money?
Tabular method

72 students and 3 teachers:
13 yuan, 8 bottles per box, 13 / 8 = 1.625
30 yuan a box of 20 bottles, 30 / 20 per bottle = 1.5
One bottle per person first: 75 * 1.625
The second 75 * 1.5

If a, B and C are three sides of triangle, simplify the square of √ (a + B-C) + the square of √ (b-c-a) + the square of √ (B + C-A)

The sum of the two sides of a triangle is greater than the third side
So a + B-C > 0
b-c-a0
So the original formula = | a + B-C | + | b-c-a | + | B + c-a|
=(a+b-c)-(b-c-a)+(b+c-a)
=a+b-c-b+c+a+b+c-a
=a+b+c

On a map with a scale of 1:6000000, the distance between a and B is 25cm. It takes an hour and 15 minutes for an airplane to fly from a to B. how many kilometers does the airplane fly per hour?

25 × 6000000 = 150000000cm = 1500km (the distance between a and B is 1500km)
1 hour 15 minutes = 1.25 hours (flight time obtained)
1500 △ 1.25 = 1200km / h (flight speed obtained)

Calculation: | 1 / 3-1 / 2 | + | 1 / 4-1 / 3 | + K + | 1 / 2004-1 / 2003|

:|1/3-1/2|+|1/4-1/3|+K+|1/2004-1/2003|
=1/2-1/3+1/3-1/4+1/4-1/5+…… +1/2003-1/2004
=1/2-1/2004
=1001/2004

The length of the railway between the two places is 490 km. The two trains leave from the two places respectively. Car a runs 80 km per hour and car B 70 km per hour. Car a goes first
Half an hour later, car B leaves. How many hours later does car B meet car a?

(490 - 80×0.5)÷（80+70）
=450÷150
=3 (hours)

① The solution of the equation of 4 (32-x) + 16 = 16 ② 84-6 (2x + 4) = 36

① 4 (32-x) + 16 = 16 is 32-x = 0, x = 32
②84-6（2x+4）=36
84-12x-24=36
12x=24
x=2

The distance between a and B is 360km. A freight car and a passenger car leave from a and B at the same time. It is known that the speed of the passenger car is 50km per hour, and the speed of the freight car is 4 / 5 of that of the passenger car. How many hours after the two cars leave, do they meet?

360÷（50+50×（4/5))=4
The two cars met four hours after they left

The decimal point of a decimal is moved one place to the right, and the new number is 38.88 compared with the original number?

Let the original decimal be x and the decimal point be moved one place to the right to get 10x, 10x-x = 38.88,
9x=38.88,
x=4.32

A freight car and a bus are running from a and B at the same time. Their speed ratio is 5:7. They meet at a distance of 30 kilometers from the midpoint. They are thousands of meters apart
Please write down the process and ideas. Every step should have ideas

30 × 2 ÷ (7-5) × (7 + 5) = 360 km
The meeting place is 30km away from the midpoint, so the express train takes two 30km more than the local train, that is, 60km, while the express train takes two more than the local train, so one is 30km. The whole journey is divided into 12 parts, so the full name is 360km