UPSC 2016 CDS II Math Paper

Let the original speed of the train be v m/minute and the time taken by it be t minutes.
∴v×t=(v+

20

100

v)×(t−20)
⇒t=1.2t−24⇒0.2t=24
⇒t=120 minutes
Trick:
If the speed is increased by

1

5

of the original speed, then to cover the same distance, time will be decreased by

1

6

of the originaltime.

1

6

of the original time
=20 minutes
∴ Original time =6×20
=120 miniute