Question 484136
sketch both equations on the same x-y plane.
.company A offers a cell phone fee of $90/mnth.
..company B costs $15per month for service and $.10 per minute.
.need to compare the 2 plans:
:
Plan A offers a flat $90 with unlimted calling, the equation for that
y = 90
:
Let t = no. of min used in Plan B
y = .10m + 15
:
Plot these two equation, should look like this, 
x axis, no of minutes 
y axis total cost per month
{{{ graph( 300, 200, -500, 1000, -40, 110, 90, .10x+15) }}}
You can see that Plan A is just a horizontal line at y=90 (Red)
Plan B cost increases with the number of minutes used
To find how many minutes for them to cost the same write
.10m + 15 = 90
.10m = 90-15
.10m = 75
m = {{{75/.1}}}
m = 750 mins which is illustrated by the graph
:
To summarize, if you use more than 750 min Plan A is best if you use less Plan B is best, right?