You need to find the unit price of each choice.
To find the unit cost, divide each price by the number of pencils.
Then the best buy is the package with the lowest unit price.
15 pencils for $4.05 ----> $0.27/pencil
12 pencils for $3.36 ----> $0.28/pencil
13 pencils for $3.25 ----> $0.25/pencil
16 pencils for $4.16 ----> $0.26/pencil
The best option is option C, 13 pencils for $3.25, which has a unit price of $0.25/pencil.