Can somebody help me. Will mark brainliest.

Average atomic mass of X= 64.25 amu

Explanation:

Abundance of X-64 = 80%

Abundance of X-65 = 15%

Abundance of X-66 = 5%

Average atomic mass = ?

Solution:  

Average atomic mass of X = (abundance of 1st isotope × its atomic mass) +(abundance of 2nd isotope × its atomic mass)  + (abundance of 3rd isotope × its atomic mass)  / 100

Average atomic mass of X= (80×64)+(15×65) +(5×66) /100

Average atomic mass of X =  5120 + 975 + 330 / 100

Average atomic mass of X  = 6425/ 100

Average atomic mass of X= 64.25 amu.

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