MathNet 2015-part-07 — 185 problems (EN–VI) Question 25

In triangle $A B C$, a point $M$ is selected in its interior so that $\angle M A B = 10^{\circ}$, $\angle M B A = 20^{\circ}$, $\angle M C A = 30^{\circ}$ and $\angle M A C = 40^{\circ}$. Determine the value of $\angle M B C$.