5. The angle at vertex $A$ of an acute-angled triangle $A B C$ is $60^{\circ}$. Through vertices $B$ and $C$, lines perpendicular to sides $A B$ and $A C$ respectively are drawn, intersecting at point $D$. Through vertex $B$, a line perpendicular to line $A D$ is drawn and intersects side $A C$ at point $M$. The lengths of segments $M A$ and $M C$ are 15 and 1, respectively. Find the length of side $B C$.

Chair of the Methodological Commission, 2021-22.