Question : Two trains 121 m and 99 m in length, respectively, are running in opposite directions, one at a speed of 40 km/hr and the other at a speed of 32 km/hr. At what time will they be completely clear of each other from the moment they meet?
Option 1: 11 sec
Option 2: 12 sec
Option 3: 13 sec
Option 4: 10 sec
Correct Answer: 11 sec
Solution :
When two trains run in opposite directions, their relative speed is the sum of their speeds.
The length of the two trains is 121 m and 99 m.
The total length = 121 m + 99 m = 220 m
The relative speed of the trains = (40 km/hr + 32 km/hr) × $\frac{5}{18}$ m/s = 20 m/s
Time taken to cross each other = $\frac{\text{Sum of the length of the trains}}{{\text{Relative speed}}}$
The time it takes for the trains to be completely clear of each other = $\frac{220}{20}$ = 11 sec
Hence, the correct answer is 11 sec.
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