Question 382304
Important equation:
  D = S X T 
where: D is the distance to be traveled,  S is the speed or rate of travel, T is the travel time.

Since both runners covered the same distance,then
  D of Henry = D of Doug

Plug in known facts to the equation above, we get
  10T = 8T + 0.5 

and the answer, 
  T = 0.25 hr = 15 min (time needed by Henry to catfch Doug.