Question -
If the speed of a train is reduced by 40 km/hr, it takes 20 minutes more to cover 1200 km. Find the speed of the train.
Options:
A .  400 km/hr
B .  360 km/hr
C .  420 km/hr
D .  380 km/hr
Answer: Option A : A Let the speed of the train be x km/hr. Time=DistanceSpeed ∴ Time takento cover 1200 km = 1200x Reduced speed = (x−40) km/hr ∴ Time takento cover 1200 km in reduced speed = 1200x−40 Relation between the time taken to cover 1200 km with speed x and with speed (x - 40) km/hr is as given below 1200x−40−1200x=13(∵20minutes=13hours) ⇒48000x(x−40)=13 ⇒144000=x2−40x ⇒x2−40x−144000=0 ⇒x2−400x+360x−144000=0 ⇒x(x−400)+360(x−400)=0 ⇒(x+360)(x−400)=0 ⇒x=−360orx=400 Since speed cannot be negative, we get x = 400 km/hr. ∴ The speed of the train is 400 km/hr.
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