The cost depends upon the number of long distance minutes, m, amy uses. write the rule that describes amy's monthly cost.

Minutes(m)              Cost
0                                  20
10                                22
20                                24
50                                30

slope formula:
m = (y1 - y2) / (x1 - x2)
m = (20 - 22) / (0 - 10) = -2/-10 = 1/5 or 0.20

y = (1/5)(x) + 20

The rule that describes Amy's monthly cost is:
m = 1/5x + 20 or m = 0.20x + 20

Where x is the number of minutes spent on the call.