the answer would be E because if you round answer E to 3mph and divide by 2 you get 1.5 and a hour is 60 minutes divide that by 2 and you get 30 minutes however you walked slightly slower so it would take 5 minutes longer.
So we want the speed of the dog which the equation got speed is;
v=d/t so using the given values we have we get;
v=1.5miles/35min but we need min in hours so by doing simple unit conversion we get;
35min(1hr/60min)=.58333hrs so now we have;
v=1.5miles/.5833hrs= 2.57mph
But to get it in form of your answer choices we can just use common sense and rule out all choices but D and E and E is too fast as it is 2.875mph and 2-4/7=2.571 which matches our answer.
So the answer is D 2 4/7mph
Hope this helped you out some and if you have any questions please feel free to ask. Thank you