If $k\geq 0$, then a $k$-edge-coloring of a graph $G$ is an assignment of colors to edges of $G$ from the set of $k$ colors, so that adjacent edges receive different colors. A $k$-edge-colorable subgraph of $G$ is maximum if it is the largest among all $k$-edge-colorable subgraphs of $G$. For a graph $G$ and $k\geq 0$, let $ν_{k}(G)$ be the number of edges of a maximum $k$-edge-colorable subgraph of $G$. In 2010 Mkrtchyan et al. proved that if $G$ is a cubic graph, then $ν_2(G)\leq \frac{|V|+2ν_3(G)}{4}$. This result implies that if the cubic graph $G$ contains a perfect matching, in particular when it is bridgeless, then $ν_2(G)\leq \frac{ν_1(G)+ν_3(G)}{2}$. One may wonder whether there are other interesting graph-classes, where a relation between $ν_2(G)$ and $\frac{ν_1(G)+ν_3(G)}{2}$ can be proved. Related with this question, in this paper we show that $ν_{k}(G) \geq \frac{ν_{k-i}(G) + ν_{k+i}(G)}{2}$ for any bipartite graph $G$, $k\geq 0$ and $i=0,1,...,k$.