$a\text{)}1{\text{dm}}^3=1000{\text{cm}}^3$     

zatem:  

$20{\text{dm}}^3=20\cdot 1000{\text{cm}}^3=20000{\text{cm}}^3$  


$1{\text{mm}}^3=0,001{\text{cm}}^3$  

zatem:

$15{\text{mm}}^3=15\cdot 0,001{\text{cm}}^3=0,015{\text{cm}}^3$  

$1{\text{m}}^3=1000000{\text{cm}}^3$  

zatem:

$0,03{\text{m}}^3=0,03\cdot 1000000{\text{cm}}^3=30000{\text{cm}}^3$  



$b\text{)}1{\text{m}}^3=1000{\text{dm}}^3$  

zatem:

$1,2{\text{m}}^3=1,2\cdot 1000{\text{dm}}^3=1200{\text{dm}}^3$  


$1{\text{cm}}^3=0,001{\text{dm}}^3$  

zatem:

$3400{\text{cm}}^3=3400\cdot 0,001=3,4{\text{dm}}^3$  


$1{\text{mm}}^3=0,000001{\text{dm}}^3$  

zatem:

$2500000{\text{mm}}^3=2500000\cdot 0,000001{\text{dm}}^3=2,5{\text{dm}}^3$