Question
Two trains 180 meters and 120 meters in length are running towards each other on parallel tracks, one at the rate 65 km/hr and another at 55 km/hr. In how many seconds will they be cross each other from the moment they meet?
Answer: Option B
$$\eqalign{
& {\text{Time taken by trains to cross each }} \cr
& {\text{other in opposite direction}} \cr
& {\text{ = }}\frac{{{l_1} + {l_2}}}{{{\text{relative speed in opposite direction}}}} \cr
& {\text{ = }}\frac{{\left( {180 + 120} \right)}}{{\left( {65 + 55} \right)}} \cr
& {\text{ = }}\frac{{300}}{{120 \times \frac{5}{{18}}}} \cr
& {\text{ = 9 seconds}} \cr} $$
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$$\eqalign{
& {\text{Time taken by trains to cross each }} \cr
& {\text{other in opposite direction}} \cr
& {\text{ = }}\frac{{{l_1} + {l_2}}}{{{\text{relative speed in opposite direction}}}} \cr
& {\text{ = }}\frac{{\left( {180 + 120} \right)}}{{\left( {65 + 55} \right)}} \cr
& {\text{ = }}\frac{{300}}{{120 \times \frac{5}{{18}}}} \cr
& {\text{ = 9 seconds}} \cr} $$
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