UPSC CDS Math Speed, Time and Distance Questions

Let the usual speed of the train be x km/hour.
Time taken by the train at usual speed – Time taken by the train at increased speed = 1 hour
⇒x2+10x−1200=0
⇒(x+40)(x−30)=0
⇒x=30 (Speed of the train cannot be negative)
Thus, the usual speed of the train is 30 km/hour.