pOH = pKb + log [salt] / [base] ---> Basic Buffer
and pH = 14 - pOH
to predict the relative concentrations of the buffer components you need to know both pKb and pH...
While in this case only pKb is given and the pH of buffer is not given. So, nothing can be concluded about the relative concentrations of CH₃NH₂ and CH₃NH₃Cl