[seragallaa]

P"cube"=P"cuboid"
F"g"/A (of cube) = F"g"/A (of cuboid) so
densityXvol.Xg/A (cube) = densityXvol.Xg (cuboid) so
10x10x10/10x10 =30x20x10/A
A(cuboide)=(30X20) so
on the side of area = 30 cm. X 20 cm