Concept: The question tests on basics of Rates(Time and Speed)
At 5:30 am, Harry has travelled for 30 minutes. Distance travelled = 60 * 0.5 = 30 miles
When Jack starts at 5:30, Harry is 30 miles away. Jack covers 90 - 60 = 30 miles over Harry every Hour.
So Jack has to cover a 30 mile gap and then another 120 miles over Harry, which is a total of 150 miles.
To this he will require 150 / 30 = 5 hours.
Therefore at 10:30 am, he will be 120 miles ahead of Harry.