In this paper, we obtain all possible general solutions of the sum form functional equations $$\begin{array}{cc}\hfill \sum _{i=1}^k\sum _{j=1}^{\ell }f\left(p_i{q}_j\right)=& \sum _{i=1}^{k}g\left(p_i\right)\sum _{j=1}^{\ell }h\left(q_j\right)\hfill \\ \hfill \text{and}\sum _{i=1}^k\sum _{j=1}^{\ell }F\left(p_i{q}_j\right)=& \sum _{i=1}^{k}G\left(p_i\right)+\sum _{j=1}^{\ell }H\left(q_j\right)+\lambda \sum _{i=1}^{k}G\left(p_i\right)\sum _{j=1}^{\ell }H\left(q_j\right)\hfill \end{array}$$ valid for all complete probability distributions $(p_1,...,p_k)$, $(q_1,...,q_{\ell })$, $k\ge 3$, $\ell \ge 3$ fixed integers; $\lambda \in ℝ$, $\lambda \ne 0$ and $F$, $G$, $H$, $f$, $g$, $h$ are real valued mappings each having the domain $I=[0,1]$, the unit closed interval.

We solve Matkowski's problem for strictly comparable quasi-arithmetic means.