Isotope 65X30 (with a mass of 65 and an abundance of 60% ): Relative mass contribution =(65*60%)=39.00 Isotope 66X30 (with a mass of 66 and an abundance of 40% ): Relative mass contribution =(66*40%)=26.40 Now, add the relative mass contributions of both isotopes: Relative atomic mass Ar=39.00+26.40=65.40 So, the relative atomic mass of element X is 65.40 . The correct option is: 65.40