As you reminded us at the end of the question:
Speed = (distance) / (time)
= (20 miles) / (60 minutes)
= (1/3) mile/minute
To convert to the units you asked for . . .
(1/3 mile/minute) · (60 minutes/hour)
= (1/3 · 60) (mile · minute) / (minute · hour)
= 20 mile/hour .