l=\frac{L}{3}3L ⇒L=3l
3(L-l)=2(L+l)-20
3(3l-l)=2(3l+l)-20
6l=8l-20
2l=20
l=10 m
L=3*10=30 m
A_{dreptunghi} =10m*30m=300 m^{2}Adreptunghi=10m∗30m=300m2
P_{dreptunghi} =2(10+30)=2*40=80 mPdreptunghi=2(10+30)=2∗40=80m
l_{patratului} =80 mlpatratului=80m
A=l²=80²m²=6400 m2