Answer:
a
     ,
  ,  
b
  
c
  
Step-by-step explanation:
From the question we are are told that 
     The  sample data is  21, 14, 13, 24, 17, 22, 25, 12
      The sample size is  n  = 8
Generally the ample mean is evaluated as 
         
         
          
Generally the standard deviation is mathematically evaluated as 
          


considering part b 
 Given that the confidence level is  90% then the significance level is evaluated as 
          
          
          
Next we obtain the critical value of   from the normal distribution table the value is
  from the normal distribution table the value is  
      
The margin of error is mathematically represented as 
       
=>    
=>     
The 90% confidence interval is evaluated as 
        
substituting values 
        
        
considering part c 
 Given that the confidence level is  95% then the significance level is evaluated as 
          
          
          
Next we obtain the critical value of   from the normal distribution table the value is
  from the normal distribution table the value is  
      
The margin of error is mathematically represented as 
       
=>    
=>     
The 95% confidence interval is evaluated as 
        
substituting values 
        
        