$\text{Có: ΔABC $\sim$ ΔMẬP}$
$\text{và: $\dfrac{AB}{MN}$ = $\dfrac{2}{3}$ (gt)}$
$\text{⇒ $\dfrac{S_{ABC}}{S_{MẬP}}$ = $k^{2}$ = ($\dfrac{x}{y})^{2}$ = $\dfrac{4}{9}$}$
$\text{⇒ $\dfrac{S_{ABC}}{20}$ = $\dfrac{4}{9}$}$
$\text{⇒ $9S_{ABC}$ = 20 . 4}$
$\text{⇒ $S_{ABC}$ = $\dfrac{80}{9}$ ($cm^{2}$)}$