Question

Two trains each of length 150 m are moving in opposite direction at 36 km/hr and 72 respectively. Time taken by them to just each other completely is ?
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Answer 1

Answer:

The time taken by them to just cross each other completely is 10 s. So, Time = 10 s. Therefore, the time taken by the two trains to just cross each other completely is 10 s.

Answer 2

$\sf\huge {\blue{\underline {\underline {Given}}}}$

• Two trains of equal lengths are crossing each other in opposite direction.

• Their speeds are 36 km/hr and 72 km/hr.

$\sf\huge {\green {\underline {\underline {To\:Find}}}}$

• Time taken to cross each other?

$\sf\huge {\red{\underline {\underline {Solution}}}}$

When two train passes a moving object (having some length) in the opposite direction, they have to cover the sum of their lengths.

Hence,

⇒ Distance to be covered = 150 m × 2

⇒ Distance to be covered = 300 m

Their speeds are opposite to each other. Hence, the relative speed will be their sum.

Unit conversion first:

$\sf 36 km/hr = 36 ×\dfrac {5}{18} m/s = 10 m/s$

$\sf 72 km/hr = 72 ×\dfrac {5}{18} m/s = 20 m/s$

Relative speed:

$\sf :\implies 10 m/s - (-20 m/s)$

$\sf :\implies 10 m/s + 20 m/s (Their\: sum)$

$\sf :\implies 30 m/s$

Then,

$\sf Time \: taken \: by\: the\: two\: trains \: to\: cross \: each \: other = \dfrac {Sum \: of\: length \: of\: trains}{Relative \: speed\: of \: trains}$

$\sf :\implies Time taken = \dfrac {300 m}{30}m/s$

⇒ Time taken = 10 s

Therefore,

${\boxed {\purple{\underline{\sf{The\: two\:trains\:crossed\:each \:other\: in \:10\: seconds.}}}}}$