Answer:
Step-by-step explanation:
Let x represent the number of calls made with plan 1 in a month.
Let y represent the number of calls made with plan 2 in a month.
Plan 1 charges $25 per month for unlimited calls. This means that the total cost of x calls made with plan 1 in a month would be $25
Plan 2 charges $15 per month plus $0.04 per call. This means that the total cost of x calls made with plan 1 in a month would be
15 + 0.04x
a) for the number of calls for which Plan 1 is more economical than Plan B, it becomes
25 < 15 + 0.04x
25 - 15 < 0.04x
10 < 0.04x
0.04x > 10
x > 10/0.04
x > 250
b) plan 1 would be more economical when the calls per month exceed 250.