[tex]\rho=\frac{m}{V}\\
\\ \Rightarrow m=\rho \cdot V\\
\\ V_{cub-total}=l^{3}=(5cm)^{3}=125cm^{3}\\
\\ V_{vid}=25cm^{3}\\
\\ V_{lemn-de-fag}=V_{cub-total}-V_{vid}\\
\\ V_{lemn-de-fag}=125cm^{3}-25cm^{3}=100cm^{3}=0,1dm^{3}=0,0001m^{3}\\
\\ m_{lemn-de-fag}= \frac{750kg}{m^{3}}\cdot 0,0001m^{3}=0,075kg=75g[/tex]
