20 minutes, or 1/3 hour
Donnie travels at 9 miles/hr.
The relationship between speed, distance, and time is Time = Distance/Speed. [from Distance = Speed*Time]
Time to bike 3 miles at 9 miles/hr is = (3 miles)/(9 miles/hr) = (1/3) hr
It took 1/3 hour, or 20 minutes, to bike the three miles, with no stops for soda and hot dog.