| A 180 m long train crosses another 270 m long train running in the opposite direction in 10.8 s. If the speed of the first train is 60 km/h then what is the speed (in km/h) of the second train? |
A) 80
B) 90
C) 150
D) Cannot be determined
E) None of the above
Correct Answer: B
Solution :
| Time taken by trains in, crossing each other |
| \[\text{=}\frac{\text{Sum}\,\,\text{of}\,\,\text{lengths}\,\,\text{of}\,\,\text{trains}}{\text{Relative}\,\,\text{speed}}\] |
| Also, \[60\,\,km/h=\frac{60\times 5}{18}=\frac{50}{3}m/s\] |
| If the speed of second train be x m/s, then |
| \[10.8=\frac{180+270}{\frac{50}{3}+x}\] |
| \[\Rightarrow \] \[10.8\left( \frac{50}{3}+x \right)=450\] |
| \[\Rightarrow \] \[180+10.8x=450\] |
| \[\Rightarrow \] \[10.8x=450-180=270\] |
| \[\Rightarrow \] \[x=\frac{270}{10.8}=25m/s\] |
| \[=25\times \frac{18}{5}km/h=90km/h\] |
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