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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Concepts:number basemodular arithmeticfactor counting

Difficulty rating: 1420

19.

In base 10,10, the number 20132013 ends in the digit 3.3. In base 9,9, on the other hand, the same number is written as (2676)9(2676)_9 and ends in the digit 6.6. For how many positive integers bb does the base-bb-representation of 20132013 end in the digit 3?3?

66

99

1313

1616

1818

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, b,b, such that 20132013 leaves a remainder of 33 when divided by b.b.

This means that bb must divide 2010.2010. Also note that b4,b \geq 4, since otherwise the remainder cannot be 3.3.

The prime factorization of 20102010 is 2010=23567. 2010 = 2 \cdot 3 \cdot 5 \cdot 67. Then, 20102010 has (1+1)4=24=16 (1 + 1)^4 = 2^4 = 16 factors. It has 33 factors less than 4,4, namely 1,2,1, 2, and 3.3. This means there are 163=1316 - 3 = 13 valid values for b.b.

Thus, C is the correct answer.

Problem 19 in Other Years