$a)20{\text{dm}}^3=20\cdot 1{\text{dm}}^3=20\cdot {\left(10\text{cm}\right)}^3=20\cdot 1000{\text{cm}}^3=20000{\text{cm}}^3$  

$\underline{20{\text{dm}}^3>2000{\text{cm}}^3}$  

 

$\underline{18{\text{cm}}^3=18\text{ml}}$  

 

$\underline{0,4\text{l}=400\text{ml}}$  

 

$0,008{\text{km}}^3=0,008\cdot 1{\text{km}}^3=0,008\cdot {\left(1000\text{m}\right)}^3=0,008\cdot 1000000000{\text{m}}^3=8000000{\text{m}}^3$  

$\underline{0,008{\text{km}}^3>80000{\text{m}}^3}$  

 

$b)50\text{l}=50{\text{dm}}^3=50\cdot 1{\text{dm}}^3=50\cdot {\left(0,1\text{m}\right)}^3=50\cdot 0,001{\text{m}}^3=0,05{\text{m}}^3$  

$\underline{50\text{l}<1{\text{m}}^3}$  

 

$300{\text{cm}}^3=300\text{ml}=0,3\text{l}$  

 

$\underline{300{\text{cm}}^3<2\text{l}}$  

$6000{\text{dm}}^3=6\cdot 1000{\text{dm}}^3=6\cdot {\left(10\text{dm}\right)}^3=6\cdot 1{\text{m}}^3=6{\text{m}}^3$  

$\underline{6000\text{l}>0,5{\text{m}}^3}$  

 

$0,9{\text{dm}}^3=900{\text{cm}}^3=900\text{ml}$  

$\underline{0,9{\text{dm}}^3>800\text{ml}}$