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NIELIT 2017 OCT Scientific Assistant A - Section A: 31

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Lets say length of platform is $\text{L metre}.$

We know that $\text{Speed} = \dfrac{\text{Distance}}{\text{Time}}$

$S_{\text{train}} = \dfrac{300}{18}\;\text{metre/sec} = \dfrac{50}{3}\;\text{metre/sec} $

$S_{\text{train}} = \dfrac{300 + L}{39}\;\text{metre/sec}$

$ \implies \dfrac{50}{3} = \dfrac{300 + L}{39}$

$\implies 300 + L = 50 \times 13 = 650$

$\implies L = 350\;\text{metre}.$

So, the correct answer is $(B).$
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