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1987 AIME Problems/Problem 8

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Problem

What is the largest positive integer n for which there is a unique integer k such that \frac{8}{15} < \frac{n}{n + k} < \frac{7}{13}?

Solution

Multiplying out all of the denominators, we get:

\begin{align*}104(n+k) &< 195n< 105(n+k)\\0 &< 91n - 104k < n + k\end{align*}

Since 91n - 104k < n + k, k > \frac{6}{7}n. Also, 0 < 91n - 104k, so k < \frac{7n}{8}. Thus, 48n < 56k < 49n. k is unique if it is within a maximum range of 112, so n = 112.

See also

1987 AIME (ProblemsResources)
Preceded by
Problem 7
Followed by
Problem 9
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Want to learn how to tackle those tough MATHCOUNTS and AMC counting and probability problems? Check out Art of Problem Solving's Introduction to Counting & Probability by David Patrick.
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