That would make sense to me, providing that's what they are getting at -I think it's a poor question and the answer they give is 4!!
Well, that's definitely a wrong answer. The only conceivable scenario I can come up with in order to reach a final answer of 4 is to consider that the aircraft burns
2100 lbs per hour, and not per half hour. This would mean that a full tank lasts 4 hours. Consequently, you would need 4 refuels:
The aircraft leaves with the tank full. This doesn't count.
1. 4 hours later, it refuels (total trip time: 4 hours).
2. 4 hours later, it refuels (total trip time: 8 hours).
3. 4 hours later, it refuels (total trip time: 12 hours).
At this point, the aircraft is out of fuel and with one hour to go still ahead. So it refuels a
4th and final time, it reaches its destination after 13 hours and 4 refuels, and the tank still holds 8400 lbs - 2100 lbs = 6300 lbs.
So, maybe they asked the question correctly ("per half hour") but then gave the answer as if it were "per hour". Their mistake, not yours.