Mixed symmetric baryon multiplets in large $N_c$ QCD: two and three flavours

We propose a new method to study mixed symmetric multiplets of baryons in the context of the $1\mathrm{/}{N}_c$ expansion approach. The simplicity of the method allows to better understand the role of various operators acting on spin and flavour degrees of freedom. The method is tested on two and three flavours. It is shown that the spin and flavour operators proportional to the quadratic invariants of SU${}_S$(2) and SU${}_F$(3) respectively are dominant in the mass formula.