Question 486371
your plans are too difficult to show simply with one linear equation each.


try this.


plan A = $60 per month and $.10 a minute.
plan B = $20 per month and $.20 a minute.


the equation for plan A would be:
y = .10x + 60
the equation for plan B would be:
y = .20x + 20


they will intersect when .10x + 60 = .20x + 20
that occurs at x = 400 minutes.


that occdurs when the total cost for each plan is equal to:
40 + 60 = 100 for plan A.
80 + 20 = 100 for plan B.


the graph of these equation is shown below:


{{{graph(600,600,-100,800,-50,250,.10x+60,.20x+20,100,50(x-400))}}}


the vertical line is at x = 400 minutes.
the horizontal line is at y = 100 dollars.


plan A is the line that intersects with the y axis at y = 60
plan B is the line that intersects with the y-axis at y = 20


plan B is cheaper when x is smaller than 400 minutes.
plan A is cheaper when x is greater than 400 minutes.