Question

7

You are comparing 3 different cell phone plans to see which is the cheapest:
Plan A costs a basic fee of $30 per month and $10 for every GB of data used
Try writing each plan as an
equation in y = mx + b form where
Plan B costs a basic fee of $80 per month with unlimited data
=GBs used and y total cost.
Plan C costs a basic fee of $50 per month and $5 for every GB of data used
You're looking to replace the m and
b in the equation. For Plan A, where
a) Write an equation for each plan that calculates the total cost (y) in terms of GB of data used (x) would the $30 go? Where would
the
Plan A:
Plan B:
Plan C:
$10 go?
b) Graph each plan on the following set of axes.
Cell Phone Plan Costs
Cost
100
c) When does Plan A cost the same as
Plan C?
80
60
d) Which plan is best if you only use 0-
4 GBs of data a month?
40:
20.
GBs of
8 10 data used
4
6
e) When does Plan B become the best
deal?

Answer 1

Plan A equation: y = 10x + 30

Plan B equation: y = x + 80

Plan C equation: y = 5x + 50

Plan A costs the same as Plan C in 4 months

Plan A is the better option if you use 0 - 4 GBs of data each month

Plan B becomes the better deal when 8 months pass

Explanation:

When does Plan A cost the same as Plan C?

Set the equations equal to each other (the cost) and solve for x (time in months)

10x + 30 = 5x + 50

5x = 20

x = 4 months

Which plan is best if you only use 0-4 GBs of data a month?

Test the maximum and minimum values to check which plan costs less

At 0 GBs of data used per month

Plan A: y = 0 + $ 30 = $30 total

Plan B: y = 0 +$80 = $80 total

Plan C: y = 0 + $50 = $50 total

At 4 GBs of data used per month

Plan A: y = $ 40 + $ 30 = $ 70

Plan B: y = $ 4 + $ 80 = $ 84

Plan C: y = $ 20 + $ 50 = $ 70

Comparing the plans at maximum and minimum amount of GBs used, only one plan has the lowest cost overall. Even though Plan C is the same price at 4 GBs as Plan A, when you use 0 GBs you will end up paying more in Plan C than Plan A. Therefore, Plan A is the better option

When does Plan B become the best deal?

When you plot the different Plans on a graph, the slope intercept of x = 7.5 (rounded up to 8) yields the lowest cost for plan B