math
posted by louise .
cellular phone companies. 1. with this plan each person is charged $19.95 per month and $0.31 per minute 2. with this plan each person is charged $40.95 per month and $0.16 per min 3. with this plan each person is charged $80.95 per month with unlimited minutes....define the variable, determine range of airtime for when each plan is least expensive

plan1 = 1995 + 31m
plan2 = 4095 + 16m
plan3 = 8095
investigate the 3 equations,
I let good ol' dependable Wolfram sketch them for you
http://www.wolframalpha.com/input/?i=plot+y+%3D+1995+%2B+31m%2C+y+%3D+4095+%2B+16m%2C+y+%3D+8095
note that the plan3 is just a horizontal straight line
to find the exact intersection points, (there are three), solve them in pairs. 
thanks!