Question
Two trains of lenths 120 m and 90 m are running with speed of 80 km/hr and 55 km/hr respectively towards each other on parallel lines. If they are 90 m apart, after how many seconds they will cross each other?
Answer: Option C
$$\eqalign{
& {\text{Relative speed}} \cr
& {\text{ = (80 + 55)km/hr}} \cr
& {\text{ = 135 km/hr}} \cr
& {\text{ = }}\left( {135 \times \frac{5}{{18}}} \right)m/\sec \cr
& = \left( {\frac{{75}}{2}} \right)m/\sec \cr
& {\text{Distance covered}} \cr
& {\text{ = (120 + 90 + 90)m}} \cr
& {\text{ = 300m}} \cr
& {\text{Required time}} \cr
& {\text{ = }}\left( {300 \times \frac{2}{{75}}} \right)\sec \cr
& = 8\sec \cr} $$
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$$\eqalign{
& {\text{Relative speed}} \cr
& {\text{ = (80 + 55)km/hr}} \cr
& {\text{ = 135 km/hr}} \cr
& {\text{ = }}\left( {135 \times \frac{5}{{18}}} \right)m/\sec \cr
& = \left( {\frac{{75}}{2}} \right)m/\sec \cr
& {\text{Distance covered}} \cr
& {\text{ = (120 + 90 + 90)m}} \cr
& {\text{ = 300m}} \cr
& {\text{Required time}} \cr
& {\text{ = }}\left( {300 \times \frac{2}{{75}}} \right)\sec \cr
& = 8\sec \cr} $$
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