Susan and john are buying cold drinks for a neighborhood picnic each person is expected to drink one can of soda susan says if you multiply the unit price by the number of people attending you will be able to determine the total cost of soda john says if you divide the cost of a 12-pack of soda by the number of sodas you will determine the total cost of sodas who is correct and why

Hello
Here we must check what each person is calculating. First, we consider Susan's method. She has suggested that we multiply the cost per soda, that is dollars/soda by the number of sodas required, we get the total cost. 
Assuming that 18 sodas are required and each costs $0.20, the total cost according to Susan is $3.60.
John suggests we divide the cost of a 12 pack of soda by the number of sodas required. Considering a 12 pack of soda costs $12 and the same amount of sodas, 18, are required, we get that each soda costs $0.66.
Looking at these answers, we see that Susan has suggested a correct method to calculate the amount of money needed to buy a number of sodas. John has suggested the amount each person would have to contribute if everyone at the party was trying to buy a 12-pack of soda; regardless of whether more or less than a 12-pack is required.

Susan is correct because to get the total cost, we multiply the unit price by the number of people attending.