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310 meter

335 meter

345 meter

350 meter

D. 350 meter

Speed = Distance/time = 300/18 = 50/3 m/sec

Let the length of the platform be x meters

then

$Dis\mathrm{tan}ce=Speed\times Time\phantom{\rule{0ex}{0ex}}x+300=\frac{50}{3}\times 39\phantom{\rule{0ex}{0ex}}\Rightarrow 3(x+300)=1950\phantom{\rule{0ex}{0ex}}\Rightarrow x=350meters$