2014 AMC 12A Problems/Problem 15
A five-digit palindrome is a positive integer with respective digits , where is non-zero. Let be the sum of all five-digit palindromes. What is the sum of the digits of ?
For each digit there are (ways of choosing and ) palindromes. So the s contribute to the sum. For each digit there are (since ) palindromes. So the s contribute to the sum. Similarly, for each there are palindromes, so the contributes to the sum.
It just so happens that so the sum of the digits of the sum is .
Notice that In fact, ordering the palindromes in ascending order, we find that the sum of the nth palindrome and the nth to last palindrome is We have palindromes, or pairs of palindromes summing to Performing the multiplication gives , so the sum .
As shown above, there are a total of five-digit palindromes. We can calculate their sum by finding the expected value of a randomly selected palindrome satisfying the conditions given, then multiplying it by to get our sum. The expected value for the ten-thousands and the units digit is , and the expected value for the thousands, hundreds, and tens digit is . Therefore our expected value is . Since the question asks for the sum of the digits of the resulting sum, we do not need to keep the trailing zeros of either or . Thus we only need to calculate , and the desired sum is .
Solution 4 (Variation of #2)
First, allow to be zero, and then subtract by how much we overcount. We'll also sum each palindrome with its . If (the line means a, b, and c are digits and ) is a palindrome, then its complement is where , , . Notice how every palindrome has a unique compliment, and that the sum of a palindrome and its complement is . Therefore, the sum of our palindromes is . (There are pairs.)
However, we have overcounted, as something like a palindrome by the problem's definition, but we've still included it. So we must subtract the sum of numbers in the form . By the same argument as before, these sum to . Therefore, the sum that the problem asks for is:
Since all we care about is the sum of the digits, we can drop the 's.
|2014 AMC 12A (Problems • Answer Key • Resources)|
|1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25|
|All AMC 12 Problems and Solutions|