Answer:
For a monthly cost of at least $7 and at most $8, you can have between 100 and 110 calling minutes.
Step-by-step explanation:
The problem states that the monthly cost of a celular plan is modeled by the following function:
[tex]C(x) = 4 + 0.10(x-70)[/tex]
In which C(x) is the monthly cost and x is the number of calling minutes.
How many calling minutes are needed for a monthly cost of at least $7?
This can be solved by the following inequality:
[tex]C(x) \geq 7[/tex]
[tex]4 + 0.10(x - 70) \geq 7[/tex]
[tex]4 + 0.10x - 7 \geq 7[/tex]
[tex]0.10x \geq 10[/tex]
[tex]x \geq \frac{10}{0.1}[/tex]
[tex]x \geq 100[/tex]
For a monthly cost of at least $7, you need to have at least 100 calling minutes.
How many calling minutes are needed for a monthly cost of at most 8:
[tex]C(x) \leq 8[/tex]
[tex]4 + 0.10(x - 70) \leq 8[/tex]
[tex]4 + 0.10x - 7 \leq 8[/tex]
[tex]0.10x \leq 11[/tex]
[tex]x \leq \frac{11}{0.1}[/tex]
[tex]x \leq 110[/tex]
For a monthly cost of at most $8, you need to have at most 110 calling minutes.
For a monthly cost of at least $7 and at most $8, you can have between 100 and 110 calling minutes.
