Total time spent: 4 hours 15 minutes (it is equivalent to 255 minutes). Let the flight time from Paris to Glasgow be X. Then the flight time from Glasgow to Paris will be (X+10).
We have an equation with one unknown variable:
X + 45 + (X+10) = 255 ⇒ X = 100
So the flight time from Paris to Glasgow is 100 minutes. If the plane leaves Paris at 08:50 then it will be in Glasgow at 10:30.