We know,

Where
D is the distance
R is the rate
T is the time
United Airlines Flight A
D = 2000 miles
T = 5 hours
So,

United Airlines Flight B
D = 2000 miles
T = 4 hours
So,

Now,
When Flight B has flown 1500 miles, 1500/500 = 3 hours have passed.
Flight A started 1 hour earlier than Flight B, so 3 + 1 = 4 hours have passed until Flight B has flown 1500 miles.
Thus,
Flight A traveled 400 miles/hr * 4 = 1600 miles in those 4 hours
How far ahead is flight A than flight B????
1600 miles - 1500 miles = 100 miles
Answer100 miles