$12a=1200m^2$

$80\%\cdot 1200m^2=0,8\cdot 1200m^2=960m^2$ $80\%\cdot 1200m^2=0,8\cdot 1200m^2=960m^2$

$\frac{1}{40}\cdot 1200m^2=30m^2$

$1200m^2-\left(960m^2+30m^2\right)=1200m^2-990m^2=210m^2$

Odp.: Dom zajmuje $210m^2.$