$\mathcal{Q}$-groups satisfy the equation $T_G(r,s)=0$

The present note shows that $\mathcal{Q}$-groups in [H. Heineken and F.G. Russo, Groups described by element numbers, Forum Math. 27 (2015), 1961–1977] are solvable groups (not necessarily nilpotent) for which the equation $T_G(r,s)=0$ is satisfied.