How do you calculate the average density of two different fluids?

-- Take a sample of the first fluid.
-- Measure its mass.
-- Measure its volume.
-- Divide its mass by its volume.
   This gives you the density of the first fluid.

-- Take a sample of the second fluid.
-- Measure its mass.
-- Measure its volume.
-- Divide its mass by its volume.
   This gives you the density of the second fluid.

You want their average ?
OK

-- Add  (Density of the first fluid) + (Density of the second fluid).
-- Divide the sum by  2 .

   Now you have the average of the two densities.

Note:
That's NOT necessarily the density of a mixture when you
pour some of fluid-1 and fluid-2 into a jar.  The density of the
fluid in the jar is going to depend on how much of each fluid is
in there. 
I started to calculate how much of each one has to be there in order
for the density of the mixture to be equal to the average of their two
densities. But then I sat up straight, asked myself "Why ? !" . 
Then I stopped, and went into the kitchen and ate some meatloaf.

B) raising the temperature and reducing the volume of the gas.|-/ 

If there was no gravity then their would be no downword force on the football to have it return the the grownd, and if there was no friction (mainly air friction) then there would be no opposing force the stop the ball from moving through the air allowing the ball to travle ∞.