Silver has an atomic mass of $107\mathrm{.}868a.$ The $Ag-109$ isotope $(108\mathrm{.}905$ amu$)$ is $48\mathrm{.}16\%.$ What is the amu of the other isotope?

$106\mathrm{.}905a$

$106\mathrm{.}908a$

$106\mathrm{.}903a$

$106\mathrm{.}911$ amu