Which equation below is a direct variation?

How many quarts of a 50% solution of acid must be added to 20 quarts of a 20% solution of acid in order to get a 40% solution of acid?

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

Awoman who is 64 inches with a shoulder width of 16 inches. write an equation relating the height h to the width w. find the height of a woman who has ashoulder width of 18.5 inches