Zamieniamy jednostki szybkości:

$108\frac{\text{km}}{\text{h}}=108\cdot \frac{1000\text{m}}{3600\text{s}}={\cancel{108}}^3\cdot \frac{10}{{\cancel{36}}^1}\frac{\text{m}}{\text{s}}=30\frac{\text{m}}{\text{s}}$   

 

$5\frac{\text{m}}{\text{s}}=5\cdot \frac{\frac{1}{1000}\text{km}}{\frac{1}{3600}\text{h}}=5\cdot \frac{3600}{1000}\frac{\text{km}}{\text{h}}=5\cdot \frac{36}{10}\frac{\text{km}}{\text{h}}=\frac{180}{10}\frac{\text{km}}{\text{h}}=18\frac{\text{km}}{\text{h}}$