you have a fixed rate of $25, meaning you get a haircut, it's $25
ANY other service is an extra $15
So, let's make the equation...
y=15x+25
Now, to just get a haircut it's $25.
But, let's say you get a haircut+shampoo
It'd be..
y=15(1)+25
y=$40
Another one..
You get a haircut+shampoo+highlight
y=15(3)+25
y=$70