Answer
$n_{air}=1.00025$
Work Step by Step
We know that
$2L=m(\frac{\lambda}{n})$
This can be rearranged as:
$\frac{n}{m}=\frac{\lambda}{2L}=constant$
$\implies \frac{n_{air}}{m_{air}}=\frac{n_{evac}}{m_{evac}}$
$n_{air}=\frac{n_{evac}}{m_{evac}}(n_{evac})$
We plug in the known values to obtain:
$n_{air}=(\frac{4001}{4000})(1)=1.00025$
