GIVEN: Distance between thief and policeman = 400 m Speed of = 30.8 km/h CONCEPT: Speed, distance and time FORMULA USED: Distance = Speed × Time CALCULATION: Let speed of thief = ‘x’ km/h Relative speed of thief and policeman = 30.8 – x Distance between thief and policeman when thief is noticed = 0.4 + x × (15 / 60) = 0.25x + 0.4 Now, 0.25x + 0.4 = (30.8 – x) × (30 / 60) ⇒ 0.5x + 0.8 = 30.8 – x ⇒ 1.5x = 30 ⇒ x = 20 ∴ Speed of thief = x = 20 km/h