Question 4
A yellow ant starts at $B$ and a black ant at $C$ on triangle $ABC$, with $AB=20$ dm, $AC=30$ dm, $BC=40$ dm. They start together at constant speeds, the yellow ant moving $B\to A\to C\to B$ and the black ant $C\to A\to B\to C$. They first meet at $A$, then next at $M\in BC$. Find $BM$ in dm.
