The ratio between the speeds of two trains is 5 ∶ 7. If the first train covers 300 km in 3 hours

The ratio between the speeds of two trains is 5 ∶ 7. If the first train covers 300 km in 3 hours, then the speed (in km/h) of the second train is

A.      100

B.      300

C.      140

D.      200

Sol:

Let the speed of the first train be 5x and the speed of the second train be 7x. 

Speed of the first train = Distance / Time = 300/3 = 100 km/hr 

According to the question:  5x = 100  ⇒ x = 20 

∴ The speed of the second train = 7x = 7 × 20 = 140 km/hr.