Question Number 120132 by Dwaipayan Shikari last updated on 29/Oct/20
$${Suppose}\:{you}\:{are}\:{in}\:{a}\:{imagnary}\:{train}\:{which}\:{travels}\:{at}\:{the}\:{half}\: \\ $$$${of}\:{the}\:{speed}\:{of}\:{light}.\:{Suppose}\:{You}\:{have}\:{a}\:{brother}\:{who}\:{is}\:\mathrm{7}\:{year} \\ $$$${smaller}\:{than}\:{you}.\:{He}\:{stands}\:{on}\:{the}\:{platform}\:{which}\:{you}\:{had}\:{left}. \\ $$$${After}\:\mathrm{1}\:{hour}\:{of}\:{travelling}\:{on}\:{the}\:{train}\:{you}\:{come}\:{back}\:{on}\:{the} \\ $$$${platform}.\:{Then}\:{you}\:{observe}\:{something}\:{strange}.\:{You}\:{can}\:{see} \\ $$$${your}\:{brother}\:{looks}\:{older}\:.\:{So}\:{what}\:{is}\:{his}\:{age}?\left({He}\:{was}\:{ten}\:{years}\right. \\ $$$$\left.{old}\right) \\ $$