# 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**.