6. A plane left minutes late due to bad weather and in order to reach the destination, km away in time, it had to increase its speed by from its usual speed. Find the usual speed of the plane.

Solution:                

Let’s assume the usual speed of the plane to be 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

This can be expressed as below:

[by factorisation method]

As the speed of the train can never be negative.

Thus, the usual speed of the train is .