Gosh I hope you didn't have to turn this in yet. I'll help you with 4 and 5.
If you want to find the unit rate, you're finding out how much money it is per ounce.
Divide your cost by the number of ounces.
So for example, 3.98 / 16.
After you get your answers, write them down. Whichever answer is the lower number is the better buy because each ounce costs less.