MD-MO 2008 Problem 3

From the vertex $A$ of the equilateral triangle $ABC$ a line is drown that intercepts the segment $[BC]$ in the point $E$ . The point $M \in (AE$ is such that $M$ external to $ABC$ , $\angle AMB = 20 ^\circ$ and $\angle AMC = 30 ^ \circ$ . What is the measure of the angle $\angle MAB$ ?