Which of the items below is not an example of a fixed cost?

At the "no mutants allowed" country farm, every sheep has 4 legs, every chicken has two legs, and all animals have only 1 head. asked to count how many sheep and chicken are in the fields, a farm boy reports back that there are 140 feet and 50 heads. how many sheep and chicken are there? there are sheep and chicken at the farm.

Find the volume of the solid bounded by the plane z=0 and the paraboloid z=1-x^2 –y^2

Jeremy wants to determine the number of solutions for the equation below without actually solving the equation.