The answer to this question must be searched in tables, books or Internet.
The result is that Magnessium has 3 natural occurring isotopes, which are Mg-24, Mg-25 and Mg-26 and their relative abundances are:
Mg-24: 78.99%
Mg-25: 10.00%
Mg-26 11.01%
Then the answer is 11.01 %