Solution:                                                           

Let’s consider the usual speed of plane as x km/hr,

Then, the increased speed of the plane is

Using, speed = distance/ time

Time taken by the plane under usual speed to cover 

Time taken by the plane under increased speed to cover

Given, that the difference in the times is . So, this can be expressed as below:       

[by factorisation method]

neglected as the speed of the aero plane can never be negative.

Therefore, the usual speed of train is .