The same as the number of two-digit numbers, since the last two digits
must the same as the first two, only reversed.
So I'll say there are 100 four-digit palindromes.
There are 90 palindromic numbers between 100 and 1000
There are 900000 of them.
90
9
1089
There are 90 palindromic numbers between 100 and 1000
infinite
90
9
There are 900000 of them.
There are not just 13 non-palindromic numbers. Most numbers are non-palindromic.
10
1089
1089
The sum of two palindromic numbers is not necessarily a palindromic number. For example, adding the palindromic numbers 121 and 131 gives 252, which is also a palindrome. However, adding 121 and 122 results in 243, which is not a palindrome. Therefore, while some sums of palindromic numbers can be palindromic, it is not a guaranteed outcome.
Twenty.
1089 palindromes