Let x be the speed of the first plane (mph), the slowest, say he's leaving from Philadelphia (P)
The speed of the second plane will be x+25
Let t=40min=2/3 hour be the time the planes pass each other.
Let d the distance from P to the point the planes pass each other. The distance from this point of meeting to Pittsburgh (T) is 275-d.
We know that the first plane went from P to the meeting point in 40 min (because they meet 40 min after their departure), at the speed x.
(you have to put the time in term of hours, because the speed is in term of hours)
The second plane went from T to the meeting point (which is 275-d distance) in 40 min too, at the speed x+25.
Now, you have to solve for x, given the system :
Can you make that ? Did you understand the way i used to come up with these equations ?