SOLUTION: The phone company Ringular has a monthly plan where a customer pays a flat monthly fee, and then a set rate per minute. If a customer uses 110 minutes, the monthly cost will be $84

Question 1165967: The phone company Ringular has a monthly plan where a customer pays a flat monthly fee, and then a set rate per minute. If a customer uses 110 minutes, the monthly cost will be $84. If the customer uses 960 minutes, the monthly cost will be $424.
Find the equation of the plan in the form y=mx+b
xis the number of monthly minutes used and
yis the total monthly cost of the Ringular plan.
Answer by Theo(13342) (Show Source): You can put this solution on YOUR website!
at 110 minutes, the monthly cost is 84 dollars.
at 960 minutes the monthly cost is 424 dollars.

straight line equation is y = mx + b.
m is the slope.
b is the y-intercept which is the value of y when x = 0.

you have two points to work with.
in (x,y) format, they are (110,84) and (960,424).
x represents the number of minutes.
y represents the total cost for that number of minutes.

m is the slope which is equal to (y1-y1)/(x2-x1).
let (110,84) = (x1,y1) and let (960,424) = (x2,y2).
m = (424-84)/(960-110) = 340/850.
simplify to get 2/5.

y = mx + b becomes y = 2/5 * x + b
to find b, replace x and y with the values from one of the points.
any one of the points will work.
i chose (x2,y2) = (960,424)
y = 2/5 * x + b becomes 424 = 2/5 * 960 + b
solve for b to get:
b = 40

y = mx + b becomes y = 2/5 * x + 40

test with the 2 known points.
with x = 110, the equation becomes y = 2/5 * 110 + 40 = 84
with x = 960, the equation becomes y = 2/5 * 960 + 40 = 424

the equation you are looking for is confirmed to be y = 2/5 * x + 40
x is the number of minutes consumed per month.
y is the total cost per month.
there is a flat monthly fee of 40 dollars.
the cost per minute is 2/5 = .40 dollars = 40 cents.
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