Alright, lets get started.
The quantity of the shampoo Jill always buys = 11.5 ounce
She is at the store buying more and sees that the bottle is now bigger and has 20% more for the same price.
Means we could find the extra shampoo she is getting by :

ounce
Means she is getting total shampoo in her new bottle =

Means she is getting total shampoo in her new bottle = 13.8 ounce : Answer
Hope it will help :)