Answer:
6.4 oz are left.
Explanation:
Start with the bottle being half full.
A bottle that is half full when it started with 16 oz is 1/2 16 oz = 8 oz
If he drinks 1/5 of the remaining amount, it means that he has 4/5 left.
4/5 * 8 = 32/5 = 6.4 oz or 6 oz and 2/5 of an ounce.