Answer
The abundance of $Ar^{40}$ is $99.6\%$
The atomic weight of $Ar^{40}$ is $39.9627\ u$
Work Step by Step
Since these three are the only stable isotopes, the abundance of $Ar^{40}$ is the complement of the others:
$100\% -0.337\%-0.063\%=99.6\%$
Argon's atomic weight is 39.948 u, which is the sum of the products of the isotopes' atomic masses by their abundances:
$39.948= 35.967545\times0.337/100+37.96732\times0.063/100+x\times99.6/100$
So the atomic weight of $Ar^{40}$ is :
$x=39.9627\ u$
