Answer:
Let's define x as the number of text messages send.
y as the number of 1,000 minutes talk per month.
(so we can write y = N/1000) where N is exactly the number of minutes that you talked that month.
> The equation for Dash is:
C1(x, y) = $80
>The equation for AB&C
C2(x,y) = $0,20*x + $55*y
> Horizon
C3(x,y) = $0.10*x + $75*y.
Now, which company you should go for?
Well, it depends on the values of x and y that you have.
if in your case you have x > y, you may want to go with Horizon, because the price per text is smaller.
if you usually talk, you may want to go with AB&C because the slope for y is smaller.
If you send a lot of texts and talk a lot, then Dash is the best option, because there is no dependence with the variables x, y
