Answer
$13cm$
Work Step by Step
The required mean free path can be calculated as follows:
$\frac{N}{V}=2000\frac{1}{m^3}$
Now $\lambda=\frac{1}{4\sqrt{2} \pi \frac{N}{V}r^2}$
We plug in the known values to obtain:
$\lambda=\frac{1}{4\sqrt{2}\pi \times 2000\times (1.5)^2}$
This simplifies to:
$\lambda=13cm$
