The speed of car is S km/hr running on the road besides a railway track. If the speed of the train is (S + 144) km/hr and car is D meter distance ahead of the engine of train. The train will pass the car in 21 seconds. If the speed of the car is 60 km/hr and the speed of the train is (S + 110) km/hr. The car is (D – 240) meter distance ahead of the engine of the train and the train will pass the car in 20 seconds. The length of the train is (D + 160) m. Then, what is the value of S? Correct Answer 58 Formula: Distance = Time × Speed Calculation: Case I: The speed of the Car = S km/hr The Speed of the train = (S + 144) km/hr Total Time = 21 seconds Total Distance = (D + D + 160) = (2D + 160) meters ∴ Relative Speed = (S + 144 – S) km/hr = 144km/hr ⇒ 144 × 5/18 m/s ⇒ 40 m/s ∴ Distance = Speed × Time ⇒ (2D + 160) = 40 × 21 ⇒ 2D = 840 – 160 ⇒ 2D = 680 ⇒ D = 340 Case II: The speed of car = 60 km/hr Speed of train = (S + 110) km/hr ∴ Relative Speed = (S + 110 – 60) = (S + 50) km/hr Total Distance = (D – 240) + (D + 160) ⇒ (2D – 80) ⇒ (680 – 80) ⇒ 600 meters Time = 20 seconds Thus, Distance = Speed × Time 600 = (S + 50) × 5/18 × 20 ⇒ (S + 50) = 6 × 18 ⇒ S + 50 = 108 ⇒ S = 108 – 50 ⇒ S = 58