RByrne,
Q: Aircraft 40 nautical miles away, at 30,000 ft descending at 3000ft per minute and travelling at 240 knots.. can it make it?
A: 3000ft per minute and starting at 30000ft means it will take the aircraft 10 mins to reach the ground. 240 knots = 4 nautical miles per minute (know your 60 time table!). In 10 mins, aircraft will travel 40 nautical miles, so it will make it (but only just!).
Hope that helps a bit...