The average atomic mass of X will be 12.8. Hence option C is corect.
However, different elements' atoms can have different quantities of neutrons in their nuclei. For instance, stable helium atoms with one or two neutrons are known, although they both have two protons. Isotopes are the several varieties of helium atoms that have differing masses (3 or 4 atomic mass units). The total number of protons and neutrons in an isotope's nucleus is referred to as the mass number. This is due to fact that each proton and neutron has a mass of the one atomic mass unit (amu). You may get the mass of the atom by multiplying the total number of protons and neutrons by the 1 amu. Each element is made up of a variety of isotopes.
Let the average atomic mass of the element x be M
M= 0.2 ×12 + 0.8× 13
=2.4+ 10.4
= 12.8
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