how to tell which is proportional to surface area and which is proportional to volume tho

you can think of it as the cost of soup is related to the amount of soup you put into the can, which is thus related to the volume of the can.

whereas the cost of tin is related to the amount of tin metal used, which is just the surface area of the can (you can ignore the thickness of the tin can)

hope this helps:)

thx :)