Question

Consider the set of all fractions $\dfrac{x}{y},$ where $x$ and $y$ are relatively prime positive integers. How many of these fractions have the property that if both numerator and denominator are increased by $1,$ the value of the fraction is increased by $10\%?$

$0$

$1$

$2$

$3$

$\text{infinitely many}$

Solution:

The condition is $\frac{x+1}{y+1}=\frac{11}{10}\cdot\frac{x}{y}.$ Cross-multiplying gives $10y(x+1)=11x(y+1)$ , or $xy+11x-10y=0$ .

Factoring by grouping after subtracting $110$ gives $(x-10)(y+11)=-110.$ Since $x,y$ are positive, the useful negative factor pairs are $(-1,110)$ , $(-2,55)$ , and $(-5,22)$ , producing $(x,y)=(9,99),(8,44),(5,11)$ .

Only $\frac{5}{11}$ has relatively prime numerator and denominator, so exactly one fraction works.

Thus, B is the correct answer.

2015 AMC 10A Problem 15

Solution:

Problem 15 in Other Years