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jmc

counting and probability junior

Problem

The number 121 is a palindrome, because it reads the same backwards as forward. How many integer palindromes are between 100 and 500?
Solution
The hundreds digit can be any one of 1, 2, 3 or 4. Whatever the hundreds digit is, that fixes what the units digit can be. Then, there are 10 choices for the middle (tens) digit. So, we can construct palindromes by choosing the digits.
Final answer
40