distance = rate*time
distance/time = rate
The race course is 3 miles around and Jeff does it twice, so the distance he goes = 2*3 miles = 6 miles.
He takes 20 minutes to go around once and 25 minutes to go around the second time, so the total time = 20 min + 25 min = 45 min.
His speed is miles per hour, not miles per minute, so convert 45 minutes to a hours:
45 min/60 min = 0.75 hours distance/time = rate = 6 miles/0.75 hours = 8 mph