Answer:
5/21 bottle
Explanation:
Your friend drinks 2/3 of a bottle of water, leaving 1/3.
You drink 5/7 of a bottle, leaving 4/7.
Find the difference between 4/7 and 1/3:
Use the LCD, which is 21: 12/21 - 7/21 = 5/21
The difference between the two amounts of water left in each bottle is 5/21 bottle.