A 3 5 7 9 11 B 4 7 10 13 16 How much is a when B = 100?

A 3 5 7 9 11 B 4 7 10 13 16 How much is a when B = 100?

If B = 100, it is (100-4) / 3 + 1 = 33, then the 33rd term of a is: (33-1) × 2 + 3 = 67, or if the first term of B subtracts 1, it is the first term of a, and if the second term of B subtracts 2, it is the second term of A

Natural numbers are arranged as follows. It is known that there is a number a in the nth row, and there is a number B in the next row (n + 1 row), and a and B are in the same column. If a + B = 391 all the time, what is n equal to?
1 2 3…… 14 15
30 29 28…… 17 16
31 32 33…… 44 45
60 59 58…… 47 46
……………………………………
Line n A ……
Line n + 1 B ……

According to the above rule, 1) if n is odd, then B = a + 25A + B = 391, so a + A + 25 = 391a = 183, B = 208, and because the difference between the third row and the first row is 30, so the difference between the third row and the first row is 30, so n = 2 * (183-3) / 30 + 1 = 13 1) if n is even, then B = a + 5A + B = 391, so a + A + 5 = 391a = 1

24 14 8 （ ）

38-24=14
24-14=10
14-8=6
Because 14-4 = 10
10-4=6
So 6-4 = 2
8-2=6

Write the natural number 1, 2, 3,..., 998999 into a number n = 1234... 998999, and find the sum of the numbers of n

9 1+2+3+...+9=45
10~19 1+0+1+1+1+2+...+9=55
20~29 2+0+2+1+2+2+...+9=65
And so on, 10 ~ 99 = 55 + 65 + 75 + 85 + 95 + 105 +... 135 = 855
99 45+855=900
100~199 1+0+0+1+0+1+...+1+9+9=1000
200~299 2+0+0+2+0+1+...+2+9+9=1100
And so on 100 ~ 999 1000 + 1100... + 1800 = 12600
999 900+12600=13500

There is a string of numbers 12345... 181920... 2022. This string of numbers is composed of natural numbers from 1 to 2022. What is the total number of 2

Consider "three digits" such as 000, 001 to 999. Among these 1000 digits, 1000 * 3 = 3000
The numbers 0, 1 and 9 appear the same number of times, 3000 / 10 = 300 times each
therefore
From 000, 001 to 999, and from 1000 to 1999, the number 2 appears 300 + 300 = 600 times
From 2000 to 2022, the number 2 appears a total of 29 times: thousand 23 + ten 3 + individual 3
Therefore, in the string of 12345... 181920... 2022, 2 appeared 629 times

Choose three numbers from the five numbers 1, 2, 3, 4 and 5 to form a natural number without repetition, then the number of such natural numbers is ()
A. 10B. 15C. 60D. 120

This is a step-by-step counting problem. First of all, choose three numbers from five numbers and arrange them in three positions. That is to say, choose three from five elements and arrange them in three positions. There are a total of A53 = 60 results, so choose C

How many numbers are there from 1 to 999? What's the number 1997?

999 numbers 1997

There is a number 123456789101112 ··· 997998999, that is, each number is from 1 to 999. Then the 666th number is

There are 99 one digit and two digit numbers, 666-99 = 567, and the 666th digit is one of the three digits
567 / 3 = 189, so 189 three digit numbers, 100101 188, 189 three digits,
The 666th number is 8

49 times 999 and 98 out of 99
How to calculate

49 × 999 and 98 / 99
＝49×（100－1/9）
＝49×100－49×1/9
= 4900-5 and 4 / 9
= 4894 5 / 9

999: 1998 times 999

Original formula = (999 + 1998 / 1999) * 999
＝（1000-1/1999）*999
＝1000*999-1/1999*999
＝999000-999/1999
= 998999 and 1000 / 1999