Computing Average Unit Costs

The total monthly operating costs of Chili To Go are:

$13,000+ $0.30X

where

X = servings of chili

(a) Determine the average cost per serving at each of the following monthly volumes: 100; 1,000; 5,000; and 10,000.

Ok, so I already figured up all of this information, my question is about the next part.

(b) Determine the monthly volume at which the average cost per serving is $0.70.

I have no idea what I'm even supposed to do with this