What is the answer for this -18x=18-18

Abby is preparing fruit punch for her party. she starts with a bowl filled with liquid that has 1/2 fruit juice and 1/2 sprite, but she decides it's too fruity. so she pours out 1/4 of the liquid, and then fills the bowl up again with sprite. what fraction of the punch is now sprite?

If f(x)=x+7 and g(x)=1/x what is (fog)(x)

How can constraints be used to model a real-world situation?