Two trains are moving towards each other at the speeds of 70 km/h and 85 km/h. Their lengths are 210 m and 255 m, respectively. How much time will they take to cross each other completely?

10.8 seconds

Relative speed ( when moves in opposite direction ) = 70 + 85 = 165 km/h

165 km/h = 155 × \(\frac{5 }{18 }\)  =  \(\frac{775 }{18 }\)  

Total distance = 210 + 255 = 465 m

Time taken =  \(\frac{465 × 18  }{775 }\)  

= 10.8 sec