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2003 AMC 8 Problems/Problem 19

Problem

How many integers between 1000 and 2000 have all three of the numbers 15, 20, and 25 as factors?

$\textbf{(A)}\ 1 \qquad \textbf{(B)}\ 2 \qquad \textbf{(C)}\ 3 \qquad \textbf{(D)}\ 4 \qquad \textbf{(E)}\ 5$

Solution 1

Find the least common multiple of $15, 20, 25$ by turning the numbers into their prime factorization. \[15 = 3 * 5, 20 = 2^2 * 5, 25 = 5^2\] Gather all necessary multiples $3, 2^2, 5^2$ when multiplied gets $300$. The multiples of $300 - 300, 600, 900, 1200, 1500, 1800, 2100$. The number of multiples between 1000 and 2000 is $\boxed{\textbf{(C)}\ 3}$.

Solution 2

Using the previous solution, turn $15, 20,$ and $25$ into their prime factorizations. \[15 = 3 * 5, 20 = 2^2 * 5, 25 = 5^2\] Notice that $1000$ can be prime factorized into: \[1000 = 2 * 2 * 2 * 5 * 5 * 5\] Using this, we can remove all the common factors of $15, 20,$ and $25$ that are shared with $1000$: \[3 * 5 * \cancel{2} * \cancel{2} * \cancel{5} * \cancel{5} * \cancel{5}\] We must also cancel the same factors in $1000$ and $2000$ to ensure that we don't exceed our range: \[1000 = 2 * \cancel{2} * \cancel{2} * \cancel{5} * \cancel{5} * \cancel{5}\]

\[2000 = 2 * 2 * 5 * \cancel{2} * \cancel{2} * \cancel{5} * \cancel{5}\] The product of the remaining factors of $1000$ is $2$, while the product of the remaining factors of $2000$ is $20$. The remaining numbers left of $15, 20$, and $25$ ($3$ and $5$) yield: \[3, 5, 15\] Thus, counting these numbers we get our answer of: $\boxed{\textbf{(C)}\ 3}$.


~Hawk2019

(Note that $3, 5,$ and $15$ are all less than $20$, but greater than $2$. Had they been larger than $20$ or less than $2$, they wouldn't have been between $1000$ and $2000$)

See Also

2003 AMC 8 (Problems • Answer Key • Resources)
Preceded by
Problem 18 		Followed by
Problem 20
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AJHSME/AMC 8 Problems and Solutions

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