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45 km/hr

50 km/hr

54 km/hr

55 km/hr

B. 50 km/hr

$Speedofthetrainrelativetoman=\left(\frac{125}{10}\right)m/sec\phantom{\rule{0ex}{0ex}}=\left(\frac{25}{2}\right)m/sec.\phantom{\rule{0ex}{0ex}}=\left(\frac{25}{2}x\frac{18}{5}\right)km/hr\phantom{\rule{0ex}{0ex}}=45km/hr.\phantom{\rule{0ex}{0ex}}Letthespeedofthetrainbexkm/hr.Then,relativespeed=(x-5)km/hr.\phantom{\rule{0ex}{0ex}}\therefore x-5=45\Rightarrow x=50km/hr.$