Argon has three naturally-occurring isotopes: 99.6% of 40Ar, with an atomic weight of 39.9624 amu, 0.337% of 36Ar, with an atomic weight of 35.9676 amu, and 0.063% of 38Ar, with an atomic weight of 37.9627 amu. Calculate the average atomic weight of Ar. Round off the answer to six significant figures. Do not include the units.