The half life period of a first order radioactive decay of an element is 5 days. The time ( in days ) taken for 8 g of this element to decay to 1 g is

$\begin {array} {1 1} (1)\;15 & \quad (2)\;25 \\ (3)\;40 & \quad (4)\;10 \end {array}$

(1) 15