1. 13 out of 49 * 21 + 28 * 13 out of 49, can it be simplified? 2. About 5 / 2 of XX road needs to be reconstructed. 30 workers are sent to carry out the construction. One third of the total length of XX road is reconstructed in the first week, and one third of the kilometer is reconstructed in the second week. How many kilometers are left to be reconstructed? (attention unit)

1. 13 out of 49 * 21 + 28 * 13 out of 49, can it be simplified? 2. About 5 / 2 of XX road needs to be reconstructed. 30 workers are sent to carry out the construction. One third of the total length of XX road is reconstructed in the first week, and one third of the kilometer is reconstructed in the second week. How many kilometers are left to be reconstructed? (attention unit)

13/49×21+28×13/49
＝13/49×（21+28）
＝13/49×49
＝13
5/2×（1-1/3）-1/3
＝5/2×2/3-1/3
＝5/3-1/3
= 4 / 3 km

Help make up 4 questions,
requirement:
1. Using sum to find unit "1"
2. Finding unit "1" with known difference
3. Using the rest to find the unit "1"
4. Seeking unit "1" by means of overproduction

1: The chicken farm has 300 hens, 40 less than the rooster. How many roosters are there?
2: The chicken farm has 300 hens, 40 more than cocks. How many cocks are there?
3: The farm raised 300 hens, 40 more than cocks, and 20 more ducks than cocks. How many ducks are there?
4: Last year, the TV station produced 200 TV sets, 10% more than this year. How many TV sets does the TV station produce this year?

It's just a formula, not a calculation
Q: there are 30 black rabbits on the grass. Please ask for white rabbits
There are five more white rabbits than one fifth of black rabbits
There are five more black rabbits than one fifth of white rabbits
The white rabbit is one fifth less than the black rabbit
There are five less black rabbits than one fifth of white rabbits
My parents are not at home, and I didn't go to class on Friday when I was sick. But our teacher is very fierce. If he doesn't do it, he will be scolded. It's better to have a process,

Hello, canjinfengli!
1. There are five white rabbits more than one fifth of the black rabbits
There are five more black rabbits than one fifth of white rabbits
The white rabbit is one fifth less than the black rabbit
There are five less black rabbits than one fifth of white rabbits

How to divide the sixth grade math score multiplication

For example: 2 / 3 × 6 / 4, the numerator can only be divided with the denominator, not the numerator and the numerator, and the denominator and the denominator. In 2 / 3 and 6 / 4, 2 and 4 are factor relations, that is, 2 × 2 = 4, 1 is written above 2, 3 and 6 are written above 4, 2 × 3 = 6, 1 is written above 3, 2 is written above 6. Because 2 × 3 = 6, 6 is connected to 3 by a three, 2 × 3, and the three in it is borrowed, leaving only 2

When a natural number greater than 1 is removed from 300245210, the remainder a, a + 2 and A-14 are obtained respectively. Then what is the natural number?

According to the meaning of the question, each number minus their corresponding remainder can be integer by this natural number, that is
300-a
245-(a+2)=243-a
210-(a-14)=224-a
300-a, 243-a, 224-a, these three numbers have the same factor
(300-a)-(243-a)=57
(243-a)-(224-a)=19
The common factor greater than 1 of 19 and 57 is only 19
So, the natural number is 19

Permutation and combination: how many natural numbers composed of 1, 2, 3 and 4 without repeated numbers?

One digit, four
Double digit, 2 out of 4, A42 = 12
Three digit, 3 out of 4, A43 = 24
Four digits, a44 = 24
So 64

The non-zero natural numbers are composed of numbers 0, 1 and 2, which can be used completely or not. They are arranged from small to large
Please write the specific approach, ask you prawns help, give high marks!
I already know the answer, but it's not 28, or I don't know how to calculate it!

The numbers composed of numbers 0, 1 and 2, arranged from small to large, can be understood as "ternary numbers". The ternary number 1010 is reduced to a decimal number: [(1 × 3 + 0) × 3 + 1] × 3 + 0 = 30, that is, the ternary number 1010 corresponds to 30 of the decimal number. In the original question, because 0 is ignored, 1010 ranks the 30th

The square number of natural number from small to large is arranged into a series of ordered sequence 149162364964 On the 11th position
The square number of natural number from small to large is arranged into a series of ordered sequence 149162364964 The number at position 11 is 9. What's the number at position 88?
I didn't write it clearly

The sequence can be divided into several parts
The first is 149, which is the square of 1.2.3. Each number has one digit, a total of three digits
And then 1625 81, is the square of 4 ~ 9, each number is 2 digits, a total of 12 digits
The third is 100121 961, 10 ~ 31 square numbers, each number has 3 digits, a total of 66 digits
There are 81 people here
Next, the square of 32 = 1024, corresponding to positions 82-85
33 square = 1089, corresponding to the 86th to 89th position
The 88th position is 8

With 0, 1, 2 three numbers can form a lot of natural numbers, which are arranged from small to large, respectively: 0, 1, 2, 10... Then 2012 is the number one

These numbers are:
0
One
1,2
Two
10,11,12；20,21,22
Six
100,101,102；110,111,112；120,121,122；
200,201,202；210,211,212；220,221,222；
Eighteen
1000,1001；1002；1010,1011,1012；1020,1021,1022；
1100,1101；1102；1110,1111,1112；1120,1121,1122；
1200,1201；1202；1210,1211,1212,1220,1221,1222；
Twenty seven
2000,2001,2002,2010,2011,
Five
The number smaller than 2012 is 1 + 2 + 6 + 18 + 27 + 5 = 59,
So 2012 ranked 60th
Solution 2
0000 is 00001, 00001 is 1,
For those less than 2000, there are three possibilities: individual, ten and hundred; for those thousand, there are two possibilities: 0 or 1
Therefore, there are 54 cases of numbers less than 2000 with 2 * (3 * 3 * 3) = 54;
There are five numbers greater than or equal to 2000 as follows:
2000,2001,2002,2010,2011,
Therefore, there are 54 + 5 = 59 numbers less than 2012, ranking 60th in 2012

There is a six digit number. The last three digits are the same. The first three digits are three consecutive natural numbers arranged from small to large,
Given that the sum of the six digits is equal to the last two digits, find the six digits

Let this number be x x + 1 x + 2 YYY;
Then 3x + 3 + 3Y = 11y, x y are all integer solutions, and x = 7; y = 3
So the number is 789333