1.  The time for each truck to cover 561 km is slightly more than  = 7 hours. I will refer to this duration as 7+ hours.


2.  When the truck "T" is going to start at 5:00 am from Winnipeg, exactly 7 trucks are just on the way from Regina to Winnipeg, 

    without counting the truck which will start at 5:00 am from Regina.

    At 5:00 am, these 7 trucks are at the distance 80, 160, 240, 320, 400, 480 and 560 kilometers from Regina.

    Our truck "T" will meet all these 7 trucks.



3.  In addition to it, on the way from W to R our truck "T" will meet all the trucks that will leave Regina at 5:00 and 

    during 7+ hours after that,  i.e.  at 5:00 am, 6:00 am, 7:00 am, 8:00 am, 9:00 am, 10:00 am, 11:00 am, and at noon. 

    There are 8 trucks in this list.



4.  In all, the truck "T" will meet  7 + 8 trucks = 15 trucks.



Answer.  On the way from W to R each mail truck will meet 15 other mail trucks .