Webwork 11 - problem 11

You have to label what those numbers represent. On a side note, the last question f'(225) = 1.2, I'm pretty sure I know the meaning of this statement, but I am unable to express what I mean to WebWork without it getting angry at me. I interpret this statement as saying that for 225 gallons of Ice Cream, you need to spend $1.2 dollars per gallon.