Hey there!
Since we don't know how many hours it took, we'll say 9 mph = 3 miles/x hours. To solve, we multiply both sides by x. 9x = 3 Dividing both sides by 9, we get that it took 3/9 hours or 1/3 hours. How many minutes are 1/3 hours? 20, so it took Derrick 20 minutes. Hope this helps! :)
Hugs!
- Nicole