24. In this question, $S_{\triangle X Y Z}$ denotes the area of $\triangle X Y Z$. In the following figure, if $D E / / B C$, $S_{\triangle A D E}=1$ and $S_{\triangle A D C}=4$, find $S_{\triangle D B C}$.
24. Ans: 12 .
$S_{\triangle A D E}: S_{\triangle C D E}=1: 3$. Hence $A E: E C=A D: D B=1: 3$. Thus $A E: A C=A D: A B=$ $1: 4$. Hence $S_{\triangle A B C}=16$ and $S_{\triangle D B C}=12$.