Question

A car covers a distance of 840 km at a uniform speed. If the speed of the car is 10 km/h more, then it takes 2 hours less to cover the same distance. Find the original speed of the car.

Answer 1

let the original speed of car be x km/hr 
now given the distance covered ny car is 840km.
therefore time taken by car = 840/x
now if speed becomes x+10
then time taken will be 8400/x+10
now according to the question 
8400/x-8400/x+10=2
840(1/x - 1/x+10)=2
8400/x^2+10x=2
2x^2+20x-8400=0
x^2+10x-4200=0
solve this equation u will get the original speed

Answer 2

The answer is 60km/hr

840/x-840/(x+10)=2
(420x+4200-420x)/x^2+10x=2
x^2+10x-4200=0
x^2+70x-60x-4200=0
(x+70)(x-60)=0
x=60 km/hr