The mean lives of an radio active substance are 1620 and 405 years for $\;\alpha-\;$emission and $\;\beta-\;$ emission respee. Find out time during which three fourth of a sample will decay if it is decaying both the $\;\alpha-\;$emission and $\;\beta-\;$ emission simultaneously

Question

$(a)\;512 years\qquad(b)\;268 years\qquad(c)\;449 years\qquad(d)\;352 years$