Verified Answer :

We have an equation: 1/? = 16/80

Cross multiply:
16? = 801
⇒ ? = 80*1/16= 5

5 fluid ounces of vinegar should be mixed with 80 fluid ounces of water~

80/16 equals to 5
5 fluid ounces of vinegar should be mixed with 80 fluid ounces of vinegar.