2017 AMC 10A Problem 25
Below is the professionally curated solution for Problem 25 of the 2017 AMC 10A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2017 AMC 10A solutions, or check the answer key.
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Difficulty rating: 2380
25.
How many integers between and inclusive, have the property that some permutation of its digits is a multiple of between and For example, both and have this property.
Solution:
We can analyze all the multiples of and see how many permutations each of them contribute. We can do this by casing on the number of unique digits in the number.
Case all the digits are the same
This cannot happen. We can see this by the divisibility rule for which says that the sum of the first and last digit minus the middle digit must be divisible by
If all the digits are the same, then the above expression evaluates to that digit, which cannot be divisible by
Case two of the digits are the same
We can split this up into the numbers that have the digit and those that don't.
There are multiples of that do not have the digit and
Each of these numbers contributes permutations, so this scenario has numbers.
There are multiples of that have the digit and
For these numbers, cannot be the hundreds digit, so each of them only contributes permutations, for a total of
Case all the digits are different
There are a total of multiples of between and The number of these with all different digits is As in case we have to specially account for the numbers with as a digit. There are and
Each of these gives us permutations, but we overcount by a factor of since flipping the first and last digits creates another number already in the set.
Therefore, these numbers provide a total of unique permutations.
There are now multiples of that we need to account for.
We know that each of these provides permutations. As above, however, note that flipping the first and last digit of any number in this set produces another number in this set.
We can see this by using the divisibility rule for If is divisible by then we have that is divisible by
This means that is divisible by which means that is also divisible by
Therefore, these numbers contribute more permutations.
Over all the cases, we have a total of numbers.
Thus, A is the correct answer.
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