2025 AIME I Problem 5
Below is the professionally curated solution for Problem 5 of the 2025 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2025 AIME I solutions, or check the answer key.
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Difficulty rating: 2510
5.
There are eight-digit positive integers that use each of the digits exactly once. Let be the number of these integers that are divisible by Find the difference between and
Solution:
The digits sum to Divisibility by requires the alternating sum of digits to be a multiple of so if the four digits in odd positions sum to then must be a multiple of Since the only possibility is each block of four positions carries digit sum The four-element subsets of with sum are eight in all, and they come in complementary pairs.
Choose which of the subsets occupies the even positions (which include the units place); the complement fills the odd positions. If that subset contains of the even digits, then the units digit can be chosen in ways, the rest of the even positions in ways, and the odd positions in ways, for numbers. Complementary subsets have -values summing to so over all choices Hence and
Problem 5 in Other Years
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