Answer
39.95 (atomic mass units)
Work Step by Step
Estimate: The vast majority of argon is $^{40}$Ar, with trace amounts of its lighter isotopes, so we can expect the average atomic mass to be slightly less than 40.
Calculate: The weighted average of isotopes is:
(.003336)(35.968 u) + (0.00063)(37.963 u) + (0.9960)(39.962 u) = 39.95 u
