Can somebody help me. Will mark brainliest.

Answer:
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.