For reference:
On the side $AC$ of a triangle $ABC$, the points $P$ and $Q$ are marked such that $AQ = PC$ $(QC > CP)$. Then $PT$ and $QR$ were traced in parallel to $AB$ ($T$ and $R$ in $BC$). If the area of the region $AHR$ is $"S"$ calculate the area of $PTHQ$ region. $(QR \cap AT=H)$ (Answer: $S$)
My progress:
$[QHTP] = [ACT]-[AHQ]-[CPT]$
$\displaystyle \frac{ATP}{AHQ}=\frac{AT.AP}{AH.HQ}$
$\displaystyle \frac{S}{[AHQ]}=\frac{HR}{HQ}$
$\displaystyle \frac{[CPT]}{[APT]}=\frac{CP}{AP}$
$\displaystyle \triangle ABC \sim \triangle CPT: \frac{CT}{CB}=\frac{CP} {AC}=\frac{PT}{AB}$
$\displaystyle \triangle CRQ \sim \triangle CPT: \frac{CT}{CR}=\frac{CP}{CQ}=\frac{PT}{PQ}$
$\displaystyle \frac{[ART]}{[AHR]}=\frac{AH}{HT}$
...?

$$ CQ:CP=QR:PT\implies AP:AQ=QR:PT \implies AP\cdot PT=AQ\cdot QR. $$ But the last equality implies that triangles $APT$, $AQR$ have the same area.