Increasing x by y% gives 12. Decreasing x by y% gives 8. Deteremine the value of x.
Hello, math123456!
I don't see how this relates to "combining functions."
It is straight algebra . . .
Increasing $\displaystyle x$ by $\displaystyle y$% gives 12.
Decreasing $\displaystyle x$ by $\displaystyle y$% gives 8.
Deteremine the value of $\displaystyle x.$
We have: .$\displaystyle \begin{array}{cccccccc}x(1+y) &=& 12 & \Longrightarrow & x + xy &=& 12 & {\color{blue}[1]} \\
x(1-y) &=& 8 & \Longrightarrow & x - xy &=& 8 & {\color{blue}[2]} \end{array}$
Add [1] and [2]: .$\displaystyle x \:=\:20 \quad\Rightarrow\quad\boxed{x \:=\:10}$