A fly is set at a point P(4m, -1m ,5m) and sees two points P1 (-1m , 2m , 0m) and P2 (1m , 1m , 4m).At time t=0 it starts flying in the plane of the three points with a constant speed of 5m/s in the direction perpendicular to the straight line P1P2,till it sees P1 and P2 are collinear at time T.Find T