Three questions of crazy 2 on three pages

Three questions of crazy 2 on three pages

In my space, I have the textbook of drawing inferences from one instance

The 29th week example 5! Today wants! To have the supplement reward score!

Example 5 the walking speed of a, B and C is 100m, 90m and 75m per minute respectively. A is at a place on the road, B and C are at B place on the road. Three people set out at the same time, and a, B and C face each other. Three minutes after a and B meet, a and C meet again. Find the distance between a and B
After a and B meet, a and C can meet again in 3 minutes. It shows that B has (100 + 75) × 3 = 525 meters more than C when a and B meet. B has 90-75 = 15 meters more than C every minute, and 525 △ 15 = 35 minutes more than C. when a and B meet in 35 minutes, it shows that the distance between a and B is (100 + 90) × 35 = 6650 meters

The average number of human hair is 120000. If it is no more than 200000 at most, there will be at least 120000 in 1.3 billion Chinese______ People have the same number of hairs

There are 200000 pieces between 1-200000, which is equivalent to 200000 drawers. 1.3-200000 = 6500, so at least 6500 people have the same number of hair

There is a rectangular refrigerator, which is 40 cm long, 30 cm wide and 35 cm deep from the inside. The water surface in the box is 10 cm high. After putting in a square iron block with 20 cm long edges, the top surface of the iron block is still higher than the water surface. How many cm high is the water surface at this time?

.
Idea: raw water volume: 40 × 30 × 10 = 12000 (cm3)
The area of refrigerator bottom after adding iron (volume is not bottom × height): 40 × 30-20 × 20 = 800 (cm2)
Water height (height is volume △ bottom): 42000 △ 800 = 15 (· cm)
A: at this time, the water surface is 15 cm high

One athlete runs 150 meters per minute in the first half of the distance and 100 meters per minute in the second half of the distance. His average speed in the whole running training is () meters per minute
A30 B125 C20
If a person's age in 2009 is the sum of the digits in the year of his birth, then the person's age is ()
A10 B19 C28
3 has seven consecutive even numbers whose sum is 280. Among the seven consecutive even numbers, the largest even number is ()
4. There are 3600 yuan coins with denominations of 10 yuan, 20 yuan, 50 yuan and 100 yuan. The number of the four coins is the same. There are () yuan coins with denominations of 10 yuan, () yuan coins with denominations of 20 yuan, () yuan coins with denominations of 50 yuan and () yuan coins with denominations of 100 yuan
5. The sum of the four natural numbers a, B, C and D is 660. The number a is four times the number B, the number B is three times the number C, and the number C is two times the number D. the number C is ()
My son is 9 years old and my father is 37 years old
The two tanks 7a and B hold 36 and 20 liters of water respectively. Now the two tanks drain water to the outside at the speed of 2 liters per minute at the same time. After () minutes, the remaining water in tank a is three times of that in tank B
Sorry, wrong number. 6 should be:
My son is 9 years old and my father is 37 years old. After () years, my father's age is three times that of my son.

B
B
Let each number be a a + 2 A + 4 A + 6 A + 8 A + 10 A + 12
So the sum is 7a + 42 = 280
A = 34, so the maximum is 46
Let's all have X sheets
So the total is (10 + 20 + 50 + 100) x = 3600
x=20
Let d be a, C be 2a, B be 6a and a be 24a
So the total is a + 2A + 6A + 24a = 660
A = 20, C = 40
After X years, the father is three times as much as the son
So 37 + x = 3 (9 + x)
2x=10
X = 5, so five years later
Let a be three times of B after X
So 36-2x = 3 (20-2x)
4x=24
x=6
Six minutes later

Two hours after Xiao Ming came out from his grandfather's house, his father came out from his home 24 kilometers away and met him again after 2.25 hours. If his father came back from his grandfather's house after 2 hours, he met him again after 1.75 hours. What's the speed of Xiao Ming and his father?
You can solve equations or use mathematical methods, but don't use bivariate equations, it's better to use elementary methods

Let Xiao Ming's and dad's speed be x and y be y
x(2+2.25)+2.25y=24
1.75x+y(2+1.75)=24
The sum of the two formulas leads to
x+y=8
The solution of x = 8-y is obtained
x=3
y=5

If the surface area of a square block is reduced by 120 square centimeters, what is the surface area of the original cuboid?
(note that it's the surface area, not the volume!)

The side length of the cube is
120 △ 2 + 3 △ 4 = 6cm
The surface area of the original cuboid is
6×6×6+120
=216+120
=336 square centimeters
Please accept if you are satisfied!

Primary school thinking exercises, who can help me do it?
On the new year's day, 31 students give each other new year cards. That is to say, one student will send one after receiving one. Is the total number of New Year cards odd or even?

Even number
30 per person
Total = 31 * 30

How to do the thinking training on page 47 of Volume 1 of grade 5?

My book is gone, you Baidu send a picture to come over, there are many versions of fifth grade volume I, why don't you write the problem directly

A and B set out from two places at the same time, facing each other, and the distance was one kilometer. A walked 6 kilometers per hour, B walked 4000 meters per hour, a took a dog, and the dog ran 10 kilometers per hour. The dog set out with a, and when he met B, he turned around and ran towards a, and when he met a, he ran towards B again, How many kilometers did the dog run? Tips. Although the time taken by the dog to meet Party A and Party B is changing, the total running time of the dog is fixed, which is equal to the time when Party A and Party B meet

From "the distance is one kilometer. A walks 6 kilometers per hour, B walks 4000 meters per hour," we can see that it takes 0.1 hour from departure to meeting
In this 0.1 hour, the dog is running continuously
It is: {1 divided by (6 + 4)} X10 = 1