Two trains `90 m` and 120 ` in length are running in opposite directions on parallel tracks with velocitye ` 72 km//h` and ` 36 km//h`. In what tiem they will complerely cross each other ?

Explanation

Relative velocity of one travin w.r.t. another
` v_r = 72 - (- 36 ) = 108 km//h = 30 ms^(-1)` ltbRgt Total distance to be coverd by each train to cross the other train is
` =90 + 120 = 2 10 m`
Time taken by each train to cross the other train
` t = ( 2 10 m)/( 30 m//s) = 7s`.