Discussion
A train overtakes two persons walking along a railway track. The first one walks at 4.5 km/hr. The other one walks at 5.4 km/hr. The train needs 8.4 and 8.5 seconds respectively to overtake them. What is the speed of the train if both the persons are walking in the same direction as the train?
Answer: D
Both walkers move the same way as the train, so the relative speeds are differences.
\[ L = (v - 4.5) \times \frac{5}{18} \times 8.4 = (v - 5.4) \times \frac{5}{18} \times 8.5 \]
\[ 8.4(v - 4.5) = 8.5(v - 5.4) \]
\[ 8.4v - 37.8 = 8.5v - 45.9 \quad\Longrightarrow\quad 0.1v = 8.1 \]
\[ v = 81 \text{ km/hr} \]
The train's length works out at 75.6 m, though the question asks only for the speed.
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