We can solve this problem by calculating the individual rate of working and equate it to their total rate of working.
If Dave can complete a sales route in 4 hours, then his working rate is

Also, if James can do it in 5 hours, then his working rate is

Let 

be the hours that both will use to complete the sales route,
Then rate at which both completes this task is 

Meaning if we add their individual rates we should get 

That is;

The LCM is 

So let us multiply through with the LCM.


We simplify to get,

Dividing through by 9 gives;


Therefore the two will complete sales route in 

hours.