Mr. Jhon left 1/2 hour before Julia and drove at 48 mph to a common destination. Julia had 3 hours to catch up with her dad. At what speed should Julia be driving?
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?