Question: Previous small study was investigating the total cost of 10 specific items of fruits and vegetables which were sold in 3 supermarkets compared to that were sold in 3 local corner shops. On average the total cost of 10 specific items of fruits and vegetables which were sold in local corner shops was £5.50. It was suggested that the total cost of those fruits and vegetables which were sold in supermarkets was 50% cheaper than the local corner shops, with a common standard deviation of 1.65. Use the PS power calculator to find out how many supermarkets and local corner shops are required (assume equal group), if we are going to conduct a similar study to investigate the difference in cost of this size, with 90% power and using a 1% significance level?