Construct a 90% confidence interval for the true average cost of a 50000 kilometre car service in Sandton , if it was found that a sample of 20 different garages in the area charged an average of...

Construct a 90% confidence interval for the true average cost of a 50000 kilometre car service in Sandton , if it was found that a sample of 20 different garages in the area charged an average of R1000 for a service. Assume a population standard deviation of R90.26