Answer:
1a
   
1b
     95% of all sample means will fall between 
1c
     
2
    
Step-by-step explanation:
From the question we are told that 
    The mean is 
    The population standard deviation is  
    The sample size is n  =  16 
Generally the standard error of the mean is mathematically represented as 
        
=>    
=>    
Generally the probability that the sample mean will be between 39 and 48 minutes is 
      
 
=> 
=> 
From the z table  the area under the normal curve to the left corresponding to  1.2  and  -2.4  is 
=> 
and  
     
So 
    
=> 
From the question we are told the confidence level is  95% , hence the level of significance is    
       
=>   
Generally from the normal distribution table the critical value  of   is  
    
Generally the margin of error is mathematically represented as  
       
=>    
  
=>    
  
Generally the  95% of all sample means will fall between
       
=>   
Generally the value which  90% of sample means is  greater than is mathematically represented 
       
=>   
=>  
Generally from the z-table  the critical  value  of  0.90  is  
       
       
=>   
Considering question 2 
  Generally we are told that the standard deviation of the mean to be one fifth of the population standard deviation, this is mathematically represented as 
          
   Generally the standard deviation of the sample mean is mathematically  represented as 
           
=>       
=>       
=>       