There are 30 boys and 20 girls in class 61. The number of boys is ()% of the number of girls, and the number of girls is ()% of the number of the whole class

There are 30 boys and 20 girls in class 61. The number of boys is ()% of the number of girls, and the number of girls is ()% of the number of the whole class

30 / 20 = 1.5 = 150%
20 / (20 + 30) = 0.4 = 40%
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There are 25 boys and 20 girls in class 61. What's the percentage of boys in the class? What's the percentage of girls in the class?

25 + 20 = 45 (person)
25÷45≈0.555＝55.5%
20÷45≈0.444＝44.4%
A: Boys account for 55.5% of the class and girls 44.4% of the class

489.096 million in scientific notation

489.096 million
=489096000
=89096 × 10

A story book has 180 pages. Xiao Ming read 1 / 6 of the whole book on the first day and 1 / 2 of the whole book on the second day. How many pages are left?
How to draw

|______ |______ |______ |______ |______ |______ |
|The first day the second day the rest-------|
Therefore, the rest are as follows:
180x(1-1/6-1/2)
=180x2/6
=60 (page)

Given the set u = R, the set a = {x | x ≤ - A-1}, B = {x | x > A + 2}, C = {x | X4}, if Cu (a ∪ b) is a subset of C, find the value set of real number a

∪ Cu (a ∪ b) is contained in C,
It is known that Cu (a) ∩ Cu (b) is contained in C
And a = {x | x ≤ A-1}, B = {x | x > A + 2}
Then there are: Cu (a) = {x | x > A-1}, Cu (b) = {x | x ≤ a + 2}
That is: Cu (a) ∩ Cu (b) = {x | A-1

It takes 10 hours for a bus to go from city a to city B, and 15 hours for a truck to go from city a to city B. at present, the two cars leave from the two cities at the same time. When they meet, the bus is still far away from city B
240 kilometers. How many kilometers is the distance between city a and city B?

The distance between the two cities is 600km, because it can be concluded from the theme that in the same journey, they spend less time
2: 3, so the speed ratio is 3:2. When the current time ratio is the same, the distance ratio is 3:2.240x1.5 = 360 (km) 240 + 360 = 600 (km)

Given the fraction 3N & # 178; + 7n + 7 / N + 2, the reduced result is an integer. What are the integers of the required integer n?

(3n^2+7n+7)/n+2=[3n（n+2）+n+7]/n+2=3n+（n+7）/（n+2）
The fraction 3N & # 178; + 7n + 7 / N + 2 is an integer
(n + 7) / (n + 2) is an integer
n=3 n=-1 n=-3 n=-7

1. The car drove from city a to city B for two fifths of the whole journey in eight hours, and at the same speed for another four hours. How many parts of the whole journey did it take
2. The audio-visual classroom of Guangming primary school purchased 72 TV sets and VCDs last month. The ratio of TV sets to VCDs is 5:4. After purchasing some VCDs this month, the ratio of TV sets to VCDs is 8:7. How many VCDs will be purchased this month? (to calculate.)

1. Suppose that the whole journey is a kilometer, and the speed per hour is x, then 8x / a = 2 / 5, then a = 20x. It means that when the same speed x is used, the whole journey is finished in 20 hours, and then the same speed is used for another 4 hours. The total journey is (8 + 4) x / 20x = 3 / 52, the number of VCDs = 72 * 4 / (5 + 4) = 32, and the number of TV stations is 72-32 = 40

The original formula = 1 + 3 + 5 +101-（2+4+6+…… 100） =（1+101）*51/2-（2+100)*50/2 =51
I can't understand the computer symbol, who can help me translate it

1、…… The regular numbers in the omitted sequence: the first is odd and the second is even
2. The asterisk denotes the multiplication sign x, which multiplies
3. The / sign stands for division
4. The original formula is equivalent to: 1-2 + 3-4 + 5-6 + 7-8 + +97-98+99-100+100
5. The final result is 51
Now you understand? This formula is a simple reasoning method to apply formula to calculate

In the words of universe, earth and moon, imagine boldly and write a paragraph

I think the universe is parallel. Our earth and moon all exist in mathematical form. It's like rolling dice ten times in a row. The probability is very small. If we roll dice all the time, will it happen again? When is it?