To find the unit prices, divide the price by the number of ounces, this way:
For the 21oz bottle that costs $8.47:
[tex]\frac{8.47}{21}=0.4033[/tex]The unit price is $0.40 per oz
For the 25oz bottle that cost $9.78:
[tex]\frac{9.78}{25}=0.3912[/tex]The unit price is $0.39 per oz.
It means that the 25oz bottle is a better deal because it has a lower unit price.
