How do you obtain the fractional abundances for two naturally occurring isotopes?

Isotopes are atoms of the same elements, and they have the same number of protons and different numbers of neutrons. In this question, copper has two isotopes. They are 63/29 Cu (having 29 protons and 34 neutrons) and 65/29 Cu (having 29 protons and 36 neutrons).

The masses of atoms are very small, and thus it is inconvenient to use “grams” as the unit of masses of atoms. The unit of the masses of atoms is “amu” (the symbol of “atomic mass unit”). By definition, the mass of one 12/5 C atom is 12.0000 amu.

Atomic weight (also known as atomic mass) of an element is the weighted average of mass (in amu) of all isotopes of this element.

The fractional abundance of an isotope means the fractions of this isotope among all isotopes in nature. For example, the fractional abundance of 35/17 Cl is 0.25 (1/4). This means that among all Cl atoms in nature, 1/4 of them are 37/17 Cl.

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Let y be the fractional abundance of 63/29 Cu, where y < 1.
Then, the fractional abundance of 65/29 Cu = 1 - y

Atomic weight of Cu (weighted average mass of the two isotopes) :
62.9298y + 64.9278(1 - y) = 63.546
62.9298y + 64.9278 - 64.9278y = 63.546
1.998y = 1.3818
y = 0.69159
1 - y = 0.30841

The fractional abundances for 63/29 Cu and 65/29 Cu are 0.69159 and 0.30841 respectively.