Answer: 1221
Explanation:
Given : A 4-digit number is a palindrome .
If it has remainder 21 when you divide it by 100, then last two digits are 21.
So required palindrome = _ _ 21
since palindrome does not change if you write it backwards, so first two numbers should be reverse of 21 = 12
hence, the complete palindrome = 1221
So the number is 1221.