UPSC CDS 2 2020 Math Paper

Let original speed of car = x km/hr
And the distance that covers = 300 km
So, time =

300

x

hr
If speed of his car =(x+15) km/hr
Then =

300

x

−1
So, distance = speed × time
⇒300=(x+15)(

300

x

−1)
⇒300=(x+15)(

300−x

x

)
⇒300x=300x+4500−x2−15x
⇒x2+15x−4500=0
⇒x=60 −75 so speed cannot negative.
Then original speed = 60 k/hr.