Sorry for being so brief. I was just going through in my mind what I would do at stage 1 if presented with such a question given the time restraints. I'll try to explain a bit more. Bear in mind though aircraft don't fly at such speeds. You're more likely to get 300mph or a speed in knots eg 480knts.
The question says a direct correlation between speed and fuel consumption.
If an aircraft goes slower it will use less fuel
So at the high speed (24166mph) it uses 5600 lbs of fuel per hour
At the lower speed (18988mph) it will use less fuel
Exactly how much less will be the ratio 18988/24166 multiplied by 5600
Given you only have a few minutes to answer this and no calculator I would round it up to roughly 20,000/25,000 multiplied by 5600 as the ratio 20000/25000 easily converts to the fraction 4/5ths or 0.8.
4/5ths of 5600 is 4480.
Multiple choice answers may be: A- 4400 B- 10,700 C - 320 D 15,800
So you would choose A.
In reality you will not get unrealistic airspeeds or anything too complicated. Things will easily divide down