A toy manufacturerer wants to see how long on average a new toy captures childrens attention.  He tests 20 children selected at random and finds that their mean attention span is 28 minutes with a standard deviation of 7 minutes.  If we assume that attention spans are normally distributed find a 95% confidence interval for the mean attention span of children playing with this new toy.  What is the lower limit of the confidence interval?  What is the upper limit of the confidence interval?