Discussion
Consider statements A and B given below and decide whether they are enough to find the time needed for two trains to cross each other when they are travelling in opposite directions and one train is moving at a speed of 86 km/hr.
Statement A – The other train is moving at 94 km/hr.
Statement B – The two trains are 250 m and 350 m long.
Answer: C
Trains moving in the same direction close at the difference of their speeds; in opposite directions, at the sum.
The crossing time is
\[ t = \frac{L_{1} + L_{2}}{v_{1} + v_{2}} \]
which needs both speeds and both lengths. One speed is already given in the question.
Statement A supplies the second speed but no lengths - the numerator stays unknown.
Statement B supplies both lengths but only one speed is known - the denominator stays unknown.
Together they give \( \tfrac{600}{180 \times 5/18} = 12 \) seconds, so both statements together are needed.
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