2014 AMC 12A Problem 20

Reflect $B$ across line $AC$ to get $B',$ and reflect $C$ across line $AB$ to get $C'.$ Then $BE=B'E$ and $CD=C'D,$ so $BE+DE+CD$ $=B'E+ED+DC',$ a broken path from $B'$ to $C'.$ This is minimized when the path is the straight segment $B'C'.$ We have $AB'=AB=10,$ $AC'=AC=6,$ and $\angle B'AC'=3\cdot40^\circ=120^\circ.$ By the Law of Cosines, $$\begin{gathered} B'C'^2=10^2+6^2\\ {}-2\cdot10\cdot6\cos120^\circ\\ =136+60\\ =196, \end{gathered}$$ so $B'C'=14.$ Thus, the correct answer is D.