Ta có: $ΔMNP \backsim ΔDEF$ theo tỉ số $k=\dfrac{2}{3}$
$=>\dfrac{MN}{DE}=\dfrac{NP}{EF}=\dfrac{PM}{DF}=\dfrac{2}{3}$
$=>\dfrac{MN}{DE}=\dfrac{NP}{EF}=\dfrac{PM}{DF}=\dfrac{MN+NP+PM}{DE+EF+DF}=\dfrac{2}{3}$
$=>\dfrac{C_{ΔMNP}}{C_{ΔDEF}}=\dfrac{2}{3}$
$=>C_{ΔDEF}=\dfrac{3}{2}.C_{ΔMNP}=90(cm)$