Order of molecular mass of solvent; given boiling point
I can see how this question is troubling.  

It don't think there is a solution.  

$K_b = \frac{RT_b^2M_w}{1000\Delta H_v}$

Where $T_b$ is boiling point, $\Delta H_v$ is the molar heat of vaporization, and $M_w$ is molecular weight of the solvent.

(see Colligative Properties by W. R. Salzman)

So you can see that knowing $K_b$ and $T_b$ is insufficient to find $M_w$, knowledge of $\Delta H_v$ is also required.  

Experimentally, from Table of cryoscopic and ebullioscopic constants you can also see there is no solution.