Respuesta :
Recall that we are told that the cell phone company charges a fixed monthly fee plus a fee that depends on how many minutes you use the phone during the month. If we let b be the fixed charged fee and m be the rate (dollars per minute) that the company charges, then, the total amount charged if we use x dollars in the month, would be
[tex]y=mx+b[/tex]which corresponds to the equation of a line. We want to calculate m, which is the slope of this line. We know that if we use the phone 260 minutes, we get a bill of 22.10. Also, we know if we spent 330, we are charged 24.55. If we translate into our variables, that means that whenever x=260 we get y=22.10 and if x=330 we get y=24.55. We can summarize this using the coordinates as (260,22.10) and (330,24.55).
Recall that, to calculate the slope that joins the points (a,b) and (c,d), we use the formula
[tex]m=\frac{d\text{ -b}}{c\text{ -a}}=\frac{b\text{ -d}}{a\text{ -c}}[/tex]In our case, we have a=260, b = 22.10, c=330, d=24.55. So, the slope of the line that we want would be
[tex]\frac{24.55\text{ - 22.10}}{330\text{ -260}}=\frac{2.45}{70}=0.035[/tex]so the company charges 0.035 dollars per minute
