The red curve extends infinitely in both directions left and right. Plugging any x value into the f(x) function leads to some corresponding y value. The other functions g(x) and p(x) have restricted domains. For instance, x = 4 would not work for g(x) or p(x), but it works for f(x).
If the ratio for water to fruit juice is 3:5, then if you divide the fruit juice by 5 to get one you must do the same on the other side, giving you 3/5(which cant be simplified further), which makes 3/5:1.
