First we have to know how much of one hour is 20 min.
To get it, we have to dividie 20min by 60 min (60 min=1hour), so
20:60=

We know that

where v is velocity, s is distance and t is time
We can modify formula to get

Now we just have to substitute our data ( t=1/3 and v=9 miles/hour)
s=

=3 miles - its the result