[Math] When the numerator of a fraction is increased by $4$, the fraction increases by $2/3$…

algebra-precalculusfractions

When the numerator of a fraction is increased by $4$, the fraction increases by $2/3$. What is the denominator of the fraction?

I tried,

Let the numerator of the fraction be $x$ and the denominator be $y$.

Accordingly, $$\frac{x+4}y=\frac xy+\frac 23$$

I am not able to find the second equation.

Best Answer

Again, you've got a fine start:

You wrote: $$\frac{x+4}y=\color{red}{\frac xy}+\color{blue}{\frac 23}\tag{1}$$

But note that $$\frac{x+4}{y} = \color{red}{\frac xy} + \color{blue}{\frac 4y}\tag{2}$$

From $(1),(2),$ it must follow that $$\color{blue}{\frac 4y = \frac 23 } \iff 2y = 4\cdot 3 = 12 \iff y = \frac{12}{2} = 6$$

So the denominator, $y$ is $6$.