A travelled for $3\text{hours}$ at a speed of $80\text{km/h}$ and then for $5\text{hours}$ at a speed of $120\text{km/h}$. What is the average speed of the journey?

The formula for average speed is given by:
$$\text{Average Speed}=\frac{\text{Total Distance Travelled}}{\text{Total Time Taken}}$$

1. Calculate the distance covered in the first leg:

$${\text{Distance}}_1={\text{Speed}}_1\times {\text{Time}}_1=80\text{km/h}\times 3\text{hours}=240\text{km}$$

2. Calculate the distance covered in the second leg:

$${\text{Distance}}_2={\text{Speed}}_2\times {\text{Time}}_2=120\text{km/h}\times 5\text{hours}=600\text{km}$$

3. Calculate total parameters:

$$\text{Total Distance}=240\text{km}+600\text{km}=840\text{km}$$

$$\text{Total Time}=3\text{hours}+5\text{hours}=8\text{hours}$$

4. Compute average speed:

$$\text{Average Speed}=\frac{840\text{km}}{8\text{hours}}=105\text{km/h}$$