if you're writing the equation using f(x), then it would be a little like that but I think it would look like this
f(x) = { 0x + 15 if x < 500 (I think you might need to throw the 0x in there but it isnt too important)
f(x) = .02x +15 if x > 500
because you cant forget the 15 because that's there regardless. and y= and f(x) = do mean the same thing yes.
And you're explanation skills are basically like mine x) But in this case x = minutes, f(x) doesnt equal minutes. f(x) is the objective and it equals the cash not the number of minutes. Just think of it like this : its basically saying that f(x) is the function of x when x equals a certain amount, or in this case, x equals the number of minutes and the function of that (x being minutes) is the total cost. Get it?
its so hard to explain Dx