A new medication is designed to lower cholesterol. The cholesterol level in a group of patients is normally distributed with a mean of 6.15 mmol/L and a standard deviation of 1.35 mmol/L. A sample of 40 people used the medication for 30 days, after which their mean cholesterol level was 5.87 mmol/L. The drug company wants to have a 95% confidence level that the drug is effective before releasing it.