luas bangun datar di samping adalah...

L1 = 10*20 = 200cm^2
L2 = 20*20 = 400cm^2
L3 (yang bertumpuk) = 5*10=50cm^2

Ltotal = L1 + L2 - 2*L3 = 200 + 400 - 2*50 = 600 - 100 = 500cm^2
mudah bukan? hehehe