Question 466040
First let's organize the information...
Plane A: Leaves at t-0, Speed=300mph, D=unknown
Plane B: Leaves at t-3, Speed=800mph, D=unknown
However, since the question asks when the one plane overtakes the other, they are essentially asking for the distances to be equal.
Here's a good way to set up these kinds of problems:
D=R*T (distance=rate*time)
Plane A:  D=300*t
Plane B:  D=800(t-3)
Now set the equation equal to each other and solve for t.
300t=800(t-3)
300t=800t-2400
-500t=-2400
t=4.8
This means that when the two planes have traveled the same distance, it has been 4.8 hours.  However, keep in mind, that this is 4.8 hours since the first plane left.  Now substitute this value for t in either equation.
D=300(4.8)=1440
1440 miles