UPSC 2015 CDS II Math Paper

Let the original speed of the car be x km/hr. Time taken by the person at the original speed - Time taken by the person at the increased speed = 1 hr
∴

300

x

−

300

x+15

=1
⇒

4500

x(x+15)

=1
⇒x2+15x−4500=0
⇒(x+75)(x−60)=0
⇒x=60km/hr (since speed cannot be negative)