Question
Two trains are moving in opposite directions @60 km/hr and 90 km/hr. Their lengths are 1.10 km and 0.9 km respectively. The time taken by the slower train to cross the faster train in second is?
Answer: Option C
$$\eqalign{
& {\text{Relative speed}} \cr
& {\text{ = (60 + 90) km/hr}} \cr
& {\text{ = }}\left( {150 \times \frac{5}{{18}}} \right){\text{m/sec}} \cr
& {\text{ = }}\left( {\frac{{125}}{3}} \right){\text{m/sec}} \cr
& {\text{Distance coverd}} \cr
& {\text{ = (1}}{\text{.10 + 0}}{\text{.9)km}} \cr
& {\text{ = 2 km}} \cr
& {\text{ = 2000 m}}{\text{}} \cr
& {\text{Required time}} \cr
& {\text{ = }}\left( {2000 \times \frac{3}{{125}}} \right)\sec \cr
& = 48{\text{ sec}}\cr} $$
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$$\eqalign{
& {\text{Relative speed}} \cr
& {\text{ = (60 + 90) km/hr}} \cr
& {\text{ = }}\left( {150 \times \frac{5}{{18}}} \right){\text{m/sec}} \cr
& {\text{ = }}\left( {\frac{{125}}{3}} \right){\text{m/sec}} \cr
& {\text{Distance coverd}} \cr
& {\text{ = (1}}{\text{.10 + 0}}{\text{.9)km}} \cr
& {\text{ = 2 km}} \cr
& {\text{ = 2000 m}}{\text{}} \cr
& {\text{Required time}} \cr
& {\text{ = }}\left( {2000 \times \frac{3}{{125}}} \right)\sec \cr
& = 48{\text{ sec}}\cr} $$
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