The day's supply is how many days the madication will last.
Since the directions are to take 1 every 8 hours and one day has 24 hours, you have to take a total of, per day:
[tex]\frac{24}{8}=3[/tex]So, each day you will have to day 3 tablets. Since you have 6 tablets, these will last 2 days, so the day's supply is 2.
