10 miles in 30 minutes would mean Julia was riding at 20mph.
5 miles in 12 minutes would mean Alex was riding at 25mph which means Alex would've had the greater average speed.
miles per hour is a good measure for this scenario because a hour is 60 minutes and the numbers 12 and 30 go into 60 cleanly.
12*5=60
30*2=60
60 minutes=1 hour
