Q. Each digit of a 9-digit number is 1. It is multiplied by itself. What is the sum of the digits of the resulting number?
(a) 64
(b) 80
(c) 81
(d) 100
Correct Answer: (c) 81
Question from UPSC Prelims 2023 CSAT
Explanation :
Each digit of a 9-digit number is 1
The 9-digit number with each digit as 1 is 111,111,111. When this number is multiplied by itself, the result is 12,345,678,987,654,321.
Sum of Digits Calculation
Now, if we sum up the digits of this number, we get:
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 = 81.
