MathNet 2016-part-06 — 184 problems (EN–VI) Question 13

$D$ is a point on side $AB$ of $\triangle ABC$, $E$ is a point on $CD$ and $F$ is a point on $CE$. The areas of triangles $AED$, $AEC$, $BFD$ and $BFC$ are $6$, $10$, $17$ and $7$, respectively. What is the area of $\triangle BEF$?