I have this statement:
A father have $x$ years old, and his son $y$ years old.
¿Within how many years the age of father will be four times the age of his son?
$x =$ Father years, $y =$ Son years
$x - y =$ Year of the birth of his son.
The father will be 4 times the age of his son, when:
$\frac{x}{4} = y$ or analogously $x = 4y$
But my try was incorrect, because the correct answer was: $\frac{x-4y}{3}$
so, how to get to that answer? and what am I doing wrong?
As pointed out the answer is $\frac{x-4y}{3}$
Given the father and son's ages as $x_1$ and $y_1$, respectively, you can ask how many years $X$ will pass for $x_2=4y_2$ where $x_2=x_1+X$ and $y_2=y_1+X$ and thus: $$x_1+X=4(y_1+X)\Rightarrow \bbox[yellow,5px]{X=\frac{x_1-4y_1}{3}}$$