2023 AIME II Problems/Problem 5

Problem

Let $S$ be the set of all positive rational numbers $r$ such that when the two numbers $r$ and $55r$ are written as fractions in lowest terms, the sum of the numerator and denominator of one fraction is the same as the sum of the numerator and denominator of the other fraction. The sum of all the elements of $S$ can be expressed in the form $\frac{p}{q},$ where $p$ and $q$ are relatively prime positive integers. Find $p+q.$

Solution

Denote $r = \frac{a}{b}$ , where $\left( a, b \right) = 1$ . We have $55 r = \frac{55a}{b}$ . Suppose $\left( 55, b \right) = 1$ , then the sum of the numerator and the denominator of $55r$ is $55a + b$ . This cannot be equal to the sum of the numerator and the denominator of $r$ , $a + b$ . Therefore, $\left( 55, b \right) \neq 1$ .

Case 1: $b$ can be written as $5c$ with $\left( c, 11 \right) = 1$ .

Thus, $55r = \frac{11a}{c}$ .

Because the sum of the numerator and the denominator of $r$ and $55r$ are the same, $a + 5c = 11a + c .$

Hence, $2c = 5 a$ .

Because $\left( a, b \right) = 1$ , $\left( a, c \right) = 1$ . Thus, $a = 2$ and $c = 5$ . Therefore, $r = \frac{a}{5c} = \frac{2}{25}$ .

Case 2: $b$ can be written as $11d$ with $\left( d, 5 \right) = 1$ .

Thus, $55r = \frac{5a}{c}$ .

Because the sum of the numerator and the denominator of $r$ and $55r$ are the same, $a + 11c = 5a + c .$

Hence, $2a = 5 c$ .

Because $\left( a, b \right) = 1$ , $\left( a, c \right) = 1$ . Thus, $a = 5$ and $c = 2$ . Therefore, $r = \frac{a}{11c} = \frac{5}{22}$ .

Case 3: $b$ can be written as $55 e$ .

Thus, $55r = \frac{a}{c}$ .

Because the sum of the numerator and the denominator of $r$ and $55r$ are the same, $a + 55c = a + c .$

Hence, $c = 0$ . This is infeasible. Thus, there is no solution in this case.

Putting all cases together, $S = \left\{ \frac{2}{25}, \frac{5}{22} \right\}$ . Therefore, the sum of all numbers in $S$ is $\frac{2}{25} + \frac{5}{22} = \frac{169}{550} .$

Therefore, the answer is $169 + 550 = \boxed{\textbf{(719) }}$ .

~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)

Note

This problem mainly comes down to noticing that $55r$ has to be simplifiable such that the numerator and denominator both change, so they potentially equal their original sum. Then you proceed with casework just as Solution 1.

~BigBrain_2009

Video Solution by The Power of Logic

https://youtu.be/qUJtReB_9sU

2023 AIME II (Problems • Answer Key • Resources)
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2023 AIME II Problems/Problem 5

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Solution

Note

Video Solution by The Power of Logic

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