if you want to know total distance traveled then
20mins at known speed and ? mins at known speed
30-20=10 so
20mns at 6mph and 10mins at 9mph
convert miles per hour to miles per minute
6mph=6mi/1hr=6mi/60mins=1mi/10mins=0.1mi/min
9mph=9mi/1hr=9mi/60mins=3mi/20mins
20mins at 6mph
distance=speed*time=20min*0.1mi/min=2mi
10 mins at 9mph
distance=speed*time=10min*3mi/20min=30/20mi=1.5mi
total is 2+1.5=3.5mi run during that 30 min duration