Let's assume the relative atomic abundance of the isotope with mass number 63 is x, and the relative atomic abundance of the isotope with mass number 65 is (1 - x).
Given that the relative atomic mass of element Y is 63.5, we can set up the following equation:
(63 * x) + (65 * (1 - x)) = 63.5
Expanding the equation:
63x + 65 - 65x = 63.5
Combining like terms:
-2x + 65 = 63.5
Subtracting 65 from both sides:
-2x = -1.5
Dividing both sides by -2:
x = 0.75
Therefore, the relative atomic abundance of the isotope with mass number 63 is 0.75, and the relative atomic abundance of the isotope with mass number 65 is (1 - 0.75) = 0.25.
To summarize, the relative atomic abundance of the isotope with mass number 63 is 0.75 (or 75%), and the relative atomic abundance of the isotope with mass number 65 is 0.25 (or 25%).