Question

A 300 m long train is moving with a uniform velocity of 90 km/h. The time taken by the train to cross another train of length 700 m moving with same speed in opposite direction is (1) 56 s (2) 68 s (3) 20 s (4) 92 s



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Answer 1

$option : \: (3) \: 20s$

Explanation:

$Time \: \: taken \: \: by \: \: the \: \: train \: \: to \: \: cross \: \: each \: \: other \\ = \boxed{ \frac{the \: \: sum \: \: of \: \: the \: \: lengths \: \: of \: \: the \: \: two \: \: trains}{sum \: \: of \: \: speed \: \: of \: \: both} } \\ = \frac{300 \: m + 700 \: m}{90 \: km {h}^{ - 1} + 90 \: km {h}^{ - 1}} \\ = \frac{1000 \: m}{180 \: km {h}^{ - 1} } \\ = \frac{1 \: km}{180 \: km {h}^{ - 1} } \\ = \frac{1}{180} \: h \\ \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: = \frac{1}{180} \times 60 \times 60s \\ = \boxed{ 20s}$