Discussion
Two trains are running in opposite directions with the same speed. If the length of each train is 120 metres and they cross each other in 12 seconds, then the speed of each train (in km/hr) is:
Answer: C
Trains moving in the same direction close at the difference of their speeds; in opposite directions, at the sum.
\[ \text{relative speed} = \frac{120 + 120}{12} = 20 \text{ m/s} \]
Both trains have the same speed \( v \), so
\[ 2v = 20 \quad\Longrightarrow\quad v = 10 \text{ m/s} = 10 \times \frac{18}{5} = 36 \text{ km/hr} \]
Forgetting to halve the relative speed gives 72 km/hr - the last option.
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