2013 AMC 10A Problem 19
Below is the professionally curated solution for Problem 19 of the 2013 AMC 10A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2013 AMC 10A solutions, or check the answer key.
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Difficulty rating: 1420
19.
In base the number ends in the digit In base on the other hand, the same number is written as and ends in the digit For how many positive integers does the base--representation of end in the digit
Solution:
Note that the units digit of represents the remainder when the number is divided by the base.
The question then boils down to finding all numbers, such that leaves a remainder of when divided by
This means that must divide Also note that since otherwise the remainder cannot be
The prime factorization of is Then, has factors. It has factors less than namely and This means there are valid values for
Thus, C is the correct answer.
Problem 19 in Other Years
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