Question 12: Write a sequence of natural numbers from 1 to 999, we get a natural number A. What is the sum of the digits of the number A?
Suppose we write the number B as the natural numbers from 000 to 999 (each number is written with 3 digits), then the sum of the digits of B is also the sum of the digits of A.
B has: 3,1000 = 3000 digits, every digit from 0 to 9 is present
3000 : 10 = 300 (times)
Sum of digits of B (also of A):
(0+1+2+…+9).300 = 45,300= 13500