X=12, -9 I hope this somewhat helps you :)
Answer:
(22.12, 27.48)
Step-by-step explanation:
Given : Significance level : 
Sample size : n= 8 , which is a small sample (n<30), so we use t-test.
Critical values using t-distribution: 
Sample mean : 
Standard deviation : 
The confidence interval for population means is given by :-

i.e. 

Hence, the 95% confidence interval, assuming the times are normally distributed.= (22.12, 27.48)
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:

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:






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:






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.
Answer:
Take a look at my notes. I hope this helps.