Maths Olympiad Prep

Track / Stage 7 / 70 of 300 #1470 of 1964

Problem 1470

National olympiad second round; IMO P1/P4
Combinatorics Difficulty 7.1 Find the answer

Many states use a sequence of three letters followed by a sequence of three digits as their standard license-plate pattern. Given that each three-letter three-digit arrangement is equally likely, the probability that such a license plate will contain at least one palindrome (a three-letter arrangement or a three-digit arrangement that reads the same left-to-right as it does right-to-left) is m/nm/n, where mm and nn are relatively prime positive integers. Find m+nm+n.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Official solution

1. Calculate the total number of possible license plates:
Each license plate consists of three letters followed by three digits.
- There are 26 possible choices for each letter.
- There are 10 possible choices for each digit.
Therefore, the total number of possible license plates is:
263×103 26^3 \times 10^3

2. Calculate the number of three-letter palindromes:
A three-letter palindrome has the form ABAABA, where AA and BB are letters.
- There are 26 choices for AA.
- There are 26 choices for BB.
Therefore, the number of three-letter palindromes is:
26×26=676 26 \times 26 = 676

3. Calculate the number of three-digit palindromes:
A three-digit palindrome has the form CDCCDC, where CC and DD are digits.
- There are 10 choices for CC.
- There are 10 choices for DD.
Therefore, the number of three-digit palindromes is:
10×10=100 10 \times 10 = 100

4. Calculate the number of license plates with at least one palindrome:
- The number of license plates with a three-letter palindrome is:
676×103 676 \times 10^3
- The number of license plates with a three-digit palindrome is:
263×100 26^3 \times 100
- The number of license plates with both a three-letter and a three-digit palindrome is:
676×100 676 \times 100

Using the principle of inclusion and exclusion, the number of license plates with at least one palindrome is:
676×103+263×100676×100 676 \times 10^3 + 26^3 \times 100 - 676 \times 100

5. Simplify the expression:
676×103+263×100676×100=676×103+263×100676×100 676 \times 10^3 + 26^3 \times 100 - 676 \times 100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
=676×103+263×100676×100 = 676 \times 10^3 + 26^3 \times 100 - 676 \times 100
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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.