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)