Verify, showing ALL calculations, whether Mr Son left his home at exactly 11:40. The following formula may be used: Distance = average speed × time.

The formula is: Distance = average speed $\times$ time.
We need to find the time, so we rearrange the formula:

$$\text{Time} = \frac{\text{Distance}}{\text{Average speed}}$$

Distance = 34 km
Average speed = 85 km/hour

$$\text{Time} = \frac{34 \text{ km}}{85 \text{ km/hour}}$$ $$\text{Time} = 0.4 \text{ hours}$$

Since there are 60 minutes in an hour:

$$\text{Time in minutes} = 0.4 \text{ hours} \times 60 \text{ minutes/hour}$$ $$\text{Time in minutes} = 24 \text{ minutes}$$

Arrival time = 12:10
Travel time = 24 minutes

$$\text{Departure time} = 12:10 - 24 \text{ minutes}$$

To subtract 24 minutes from 12:10, we can think of 12:10 as 11 hours and 70 minutes (since 1 hour = 60 minutes, 12:10 = 11:70).

$$\text{Departure time} = 11:70 - 00:24$$ $$\text{Departure time} = 11:46$$

The calculated departure time is 11:46.
The statement says Mr. Son left his home at exactly 11:40.
Since $11:46 \neq 11:40$, Mrs. Son's statement is not valid.

$$\text{Mr. Son's statement is } \boxed{\text{not valid}}$$