If Deep had walked 20 km/h faster he would have saved 1 hour in the distance of 600km. What is the usual speed of Deep?

Question: If Deep had walked 20 km/h faster he would have saved 1 hour in the distance of 600km. What is the usual speed of Deep?

Solution:
Let the original speed be x km/h,
Then
600/x - 600/(x + 20) = 1
⇒ 600[{(x + 20) - x}/x(x + 20)] = 1
⇒ x² + 20x - 12000 = 0
⇒ x² + 120x - 100x - 12000 = 0
⇒ (x + 120)(x - 100) = 0
∴ x = - 120, 100 [- 120 not acceptable]

Therefore, original speed = 100 km/h