A basic cellular phone plan costs $4 per month for 70 calling minutes. Additional time costs $0.10 per minute. The formula C= 4+0.10(x-70) gives the monthly cost for this plan, C, for x calling minutes, where x>70. How many calling minutes are possible for a monthly cost of at least $7 and at most $8?

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.