Question 63435
Distance covered by the plane moving north  in 2 hrs = 150 * 2
                                                     =  300 miles

Distance covered by the plane moving west in 2 hrs = 200 * 2
                                                     =  400 miles
So at 3 PM both the planes would have covered the above distances.

We know that North and West are perpendicular to each other.

So by Pythagorean theorem,

distance between them = sqrt(300^2  + 400^2)

                      = sqrt(900 + 1600)

                      = sqrt(2500)
                      = 50

THus they are 50 miles at 3 PM

Good luck!!!