Can someone answer a physics question about the speed of molecules

Here is a picture of the question that is much easier to read: http://imageshack.us/photo/my-images/819/physicspic.jpg/

This is the Question:

Two gases A and B are held at the same temperature. One of the gases is diatomic, and the other is not. It is found that the ratio of the rms speeds of the two gases is:
V(a)/V(b) =0.8542, where V is velocity and () denotes a identifying subscript.

Find the most likely composition of A and B. That is, what are the two gases?
For the purpose of this exercise, assume the gases are pure, and are composed of the most common isotope.

V(a)/V(b) = sqrt (M(b)/M(a)) Where M is molecular weight.
Mb = 0.73 Ma.
Monoatomic gases = He Ne Ar Kr Xe Rn
Diatomic gases = H2, N2, O2, Cl2, F2

This molecular weights satisfy the Xenon (54) and Flourine (38) composition