Answer:
$0.69 cents
Explanation:
To find the cost of one bottle you would have to divide $13.11 (the total price) by 19 (the quantity she got for that price).
By doing this you will be able to find the price of one bottle; 69 cents
To prove this concept you can multiple the 69 cents by 19 to see if you get back the original price that was given.