speed = distance / time
Give it a shot...
No, you want to find its complete answer. That's a completely different thing.
Julia drove for 3 hours and left 1/2 hour after John. John drove for 3 and 1/2 hours.
John drove for 3 and 1/2 hours at 48 mph. How far did he go?
Julia drove that same distance in 3 hours. How fast did she drive?