Answer
$283m/s$
Work Step by Step
The rms speed of the argon atoms can be determined as
$v_{rms, Ag}=\sqrt{\frac{M_{N_e}}{M_{Ag}}}v_{rms, N_e}$
We plug in the known values to obtain:
$v_{rms}=400\sqrt{\frac{20}{40}}$
This simplifies to:
$v_{rms}=283m/s$
