Difference between revisions of "1991 AIME Problems/Problem 5"
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== Problem == | == Problem == | ||
− | Given a [[rational number]], write it as a [[fraction]] in lowest terms and calculate the product of the resulting [[numerator]] and [[denominator]]. For how many rational numbers between 0 and 1 will | + | Given a [[rational number]], write it as a [[fraction]] in lowest terms and calculate the product of the resulting [[numerator]] and [[denominator]]. For how many rational numbers between 0 and 1 will <math>20_{}^{}!</math> be the resulting [[product]]? |
== Solution == | == Solution == |
Revision as of 20:53, 22 February 2011
Problem
Given a rational number, write it as a fraction in lowest terms and calculate the product of the resulting numerator and denominator. For how many rational numbers between 0 and 1 will be the resulting product?
Solution
If the fraction is in the form , then and . There are 8 prime numbers less than 20 (), and each can only be a factor of one of or . There are ways of selecting some combination of numbers for ; however, since , only half of them will be between . Therefore, the solution is .
See also
1991 AIME (Problems • Answer Key • Resources) | ||
Preceded by Problem 4 |
Followed by Problem 6 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |