(Background)A patient visits her doctor with concerns about her blood pressure. If the systolic blood pressure exceeds 150, the patient is considered to have high blood pressure and medication may be prescribed. The problem is that there is a considerable variation in a patients systolic blood pressure readings during a given day.

Question

How many measurements would be required so that the probability is at most 5% of failing to detect that the patient has high blood pressure?

I was using the formula(150-155)/(20/sqrt N, but I am not sure how to find N)