GIVEN: Speed of A = 30 km/h Speed of B = 45 km/h CONCEPT: Speed, time and distance FORMULA USED: Distance = Speed × Time CALCULATION: Let length of race = D km Time taken by B to finish the race = (D/45) hours Total distance covered by A = (D – 4) km Time taken by A to cover ‘D – 4’ km = (D – 4)/30 = D/45 3D – 12 = 2D ⇒ D = 12 ∴ Length of race = 12 km