Question 484136
company A charges $90.00 per month flat fee.
company B charges $15.00 pr month plus $.10 per minute.
equation for company A is:
y = 90
equation for company B is:
y = .10*x + 15
you can graph both equations.
looks like this:
{{{graph(400,400,-100,1000,-10,100,90,.10x+15)}}}
the plan with the flat rate is more expensive up to about x minutes.
you can solve for the exact value by solving the 2 equations simultaneously.


the 2 equations are:
y = 90
y = .10x + 15
since they both equal to y, then set them equal to each other to get:
90 = .10x + 15
subtract 15 from both sides of the equation to get:
75 = .10x
divide both sides of the equation by .10 to get:
x = 750
the break even point is at 750 minutes.
the graph has been modified to provide a horizontal line at y = 90 and a vertical line at x = 750.
the intersection of the horizontal line and the vertical line should be the break even point.
{{{graph(400,400,-100,1000,-10,100,90,.10x+15,10*(x-750))}}}
the horizontal line at y = 90 is the equation for Company A.
The slanted line is the equation for Company B.
It intercepts the y axis at x = 15 which is the cost to the customer with 0 minutes used.
The vertical line shows you that the intersection of the graph of the equation for Company A and the graph of the equation for Company B occurs at x = 750 which is the break even point.
the value of x in the graph is the number of minutes used.
the value of y in the graph is the cost for the number of minutes used.
you can see that the cost for company A customer is $90.00 regardless of the number of minutes they used, while the cost for company B customer has a fixed component ($15.00) and a variable component (10 cents a minute).