A journey consists of three parts. Part 1: 2 hours at 60 km/h. Part 2: 30 minutes at 10 km/h. Part 3: 40 minutes at an unknown speed. Calculate the total distance and total time.

The formula for distance is $\text{Distance} = \text{Speed} \times \text{Time}$.

First, convert 30 minutes to hours.

Now, calculate the distance:

Time for the first part ($T_1$) = 2 hours
Time for the rest ($T_{\text{rest}}$) = 40 minutes $= \frac{40}{60} \text{ hours} = \frac{2}{3} \text{ hours}$
Time for the second part ($T_2$) = 30 minutes $= \frac{30}{60} \text{ hours} = 0.5 \text{ hours} = \frac{1}{2} \text{ hours}$

To add these, find a common denominator, which is 6:

As a decimal, this is approximately:

So, the total time taken is $\boxed{\frac{19}{6} \text{ hours}}$.

Total distance ($D_{\text{total}}$) = 125 km
Total time ($T_{\text{total}}$) = $\frac{19}{6}$ hours

The formula for average speed is $\text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}}$.

As a decimal, this is approximately:

So, the average speed is $\boxed{\frac{750}{19} \text{ km/h}}$.