average atomic mass of an element=Atomic mass isotope₁* percent abundance isotope₁ + Atomic mass isotope₂* percent abundance isotope₂+Atomic mass isotope₃* percent abundance isotope₃....
average atomic mass=39.96 u*0.9959+37.96 u*0.00065+
35.97 u*0.00366≈39.95 u
Answer: the average atomic mass of this element is 39.95 u
