Two trains start running towards each other from two places 396 km apart.  The ratio of speeds of trains is 7 : 11 and the speed of first train is 77 km/h.  After how many minutes will both the trains meet each other?

120

              1st    :     2nd

Speed =   7     :      11

ATQ,

7R = 77 ⇒ 1R = 11

⇒ Relative speed = (7 + 11)R = 18R = 18 × 11 = 198 km/hr

Time = \(\frac{396}{198}\) = 2 h = 120 mins.