Q. If N² = 12345678987654321, then how many digits does the number N have?
(a) 8
(b) 9
(c) 10
(d) 11
Correct Answer: (b) 9.
UPSC Prelims 2025 CSAT
Explanation :
1. Observe the given square is a 17-digit palindrome:
12345678987654321.
2. Recall a well-known fact about “repunit” numbers (numbers consisting only of 1’s):
(111111111)² = 12345678987654321.
3. Thus N = 111 111 111, which clearly has 9 digits.
