To solve this problem you must apply the proccedure shown below:
1. You have that
varies jointly as
and
and inversely as the product of
and
. Therefore, you can write the following equation, where
is the constant of proportionality:

2. Now, you must solve for the constant of proportionality, as following:

3. Susbtiute values:


4. Substitute the value of the constant of proportionality into the equation:

The answer is: 
The diffusion time of the red blood cell to take oxygen through the membrane is 25uS.
The small capillaries in the lungs are in close contact and it takes a red blood cell 0.5 seconds to squeeze through the capillary.
The diffusion time for the oxygen across the 1 um thick membrane separating the air is given by,
t = x²/2D
Where,
t is the diffusion time,
x is the thickness of the membrane,
D is the diffusion coefficient.
Putting values,
t = (1 x 10⁻⁹)²/2 x 2 x 10⁻¹¹
t = 10⁻⁷/4
t = 25 x 10⁻⁹ seconds.
t = 25 uS.
so, the diffusion time is 25uS.
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R2+15r+36=0 the answer is r=6