2018 AMC 8 Problem 18

Below is the video solution and professionally curated solution for Problem 18 of the 2018 AMC 8, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2018 AMC 8 solutions, or check the answer key.

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Concepts:prime factorizationfactor counting

Difficulty rating: 1170

18.

How many positive factors does 23,232 have?

9 9

12 12

28 28

36 36

42 42

Video solution:
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Written solution:

Begin by finding the prime factorization of 23232.23232. To do this, we repeatedly factor out the smallest prime factor from the number, a process that terminates when the number is a prime number. This process is outlined below: 23232=211616=225808=232904=241452=25726=26363=263121=263112\begin{align*}23232 &= 2\cdot 11616\\ &= 2^2\cdot 5808\\ &= 2^3\cdot 2904\\ &= 2^4\cdot 1452\\ &= 2^5\cdot 726\\ &= 2^6\cdot 363\\ &= 2^6\cdot 3 \cdot 121\\ &=2^6\cdot 3\cdot 11^2 \end{align*} An arbitrary factor of 2323223232 can be created by taking the product of any number of prime factors. More explicitly, as 2323223232 can be represented p1e1p2e2pmemp_1^{e_1} p_2^{e_2} \cdots p_m^{e_m} where pp is a prime number, each factor has (e1+1)(em+1)(e_1+1) \cdots (e_m+1) options of prime factorizations to choose from, and thus, there are (e1+1)(em+1)(e_1+1) \cdots (e_m+1) factors. Plugging in values, we can see that there are (6+1)(1+1)(2+1)=723=42\begin{align*}(6+1)(1+1)(2+1) &= 7\cdot2\cdot3\\ &= 42 \end{align*} factors of 23232.23232.

Thus, E is the correct answer.

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