Answer:  
OR
(15.102, 15.498)
Step-by-step explanation:
The formula to find the confidence interval is given by :-
 is given by :-

, where n is the sample size 
 = sample standard deviation.
 = sample standard deviation.
 = Sample mean
= Sample mean
 = Two tailed z-value for significance level of
 = Two tailed z-value for significance level of  .
 .
Given : Confidence level = 95% = 0.95
Significance level = 
 
 

sample size : n= 250 , which is extremely large ( than n=30) .
So we assume sample standard deviation is the population standard deviation.
thus , 
By standard normal  distribution table ,
Two tailed z-value for Significance level of 0.05 : 

Then, the 95% confidence interval for the mean credit hours taken by a student each quarter :-

Hence, the mean credit hours taken by a student each quarter using a 95% confidence interval. =(15.102, 15.498)