Let's call x the number of months you have a membership, and y the total cost of the membership.
We want to express the total cost function in slope-intercept form:
y = mx + b
What is b, the y intercept? It is the cost you pay aside from your monthly fee. This is the initital membership fee of 20 dollars.
What is m, the slope? This is the amount that the total cost increases by in each month, or the monthly. The slope, therefore, is 15 dollars.
y = 15x + 20
How much will it cost for a year's membership? We know that there are 12 months in a year, so let's plug in 12 for x and solve for y:
y = 15 (12) + 20
y = 180 + 20
y = 200
One year's membership costs 200 dollars.
Graphing the equation:
One easy way to graph a linear function, draw a straight line between the x-intercept and the y-intercept that continues in both directions.
The y-intercept, as we know, is 20, so that is your first point.
To find the x-intercept:
0 = 15(x) + 20
15x = -20
x = -20/15 = -4/3
Therefore, drawing a straight line that goes through the points (-4/3, 0) and
(0, 20) will provide a graph of the equation.
