A phone company has a monthly data plan where a customer pays flat monthly fee of $ 15 and then a certain amount of money per gigabyte (GB) of data used on the phone. If a customer uses 3 GB, the monthly cost will be $ 24. If the customer uses 7 GB, the monthly cost will be $ 36.

Find a linear equation for the monthly cost of the data plan as a function of x, the number of MB used.

Step-by-step explanation:

let x represent the number of MB used in a month.

Let y represent the monthly cost of using x MB.

If we plot y on the vertical axis and x on the horizontal axis, a straight line would be formed. The slope of the straight line would be

Slope, m = (36 - 24) / (7 - 3)

m = 12/4 = $3

We would represent the linear equation in the slope - intercept form which is expressed as

y = mx + c

Where

c = y intercept

m = slope

At the y intercept, x = 0. Therefore, the y intercept is the flat monthly fee of $15. It is charged whether MB is used or not. Therefore, the linear equation is

y = 3x + 15

Or

C (x) = 3x + 15