A person travels $3.5\text{km}$ from place A to place B. Out of this distance, he travels $\frac{5}{3}\text{km}$ on bicycle, $\frac{7}{6}\text{km}$ on scooter and the rest on foot. What portion of the whole distance does he cover on foot?

The total distance traveled is $3.5\text{km}=\frac{35}{10}=\frac{7}{2}\text{km}$.
Distance covered by bicycle and scooter combined:

$${\text{Distance}}_{\text{bike + scooter}}=\frac{5}{3}+\frac{7}{6}$$

Find a common denominator, which is $6$:

$${\text{Distance}}_{\text{bike + scooter}}=\frac{10}{6}+\frac{7}{6}=\frac{17}{6}\text{km}$$

Distance covered on foot:

$${\text{Distance}}_{\text{foot}}=\text{Total Distance}-{\text{Distance}}_{\text{bike + scooter}}$$

$${\text{Distance}}_{\text{foot}}=\frac{7}{2}-\frac{17}{6}$$

Find a common denominator, which is $6$:

$${\text{Distance}}_{\text{foot}}=\frac{21}{6}-\frac{17}{6}=\frac{4}{6}=\frac{2}{3}\text{km}$$

Portion of the whole distance covered on foot:

$$\text{Portion}=\frac{{\text{Distance}}_{\text{foot}}}{\text{Total Distance}}=\frac{\frac{2}{3}}{\frac{7}{2}}=\frac{2}{3}\times \frac{2}{7}=\frac{4}{21}$$