The relative abundances of the three isotopes must be equal to 1. Suppose x is the abundance of 25 mg, and y is the abundance of 26 mg, then,
x + y + 0.79 = 1 --> eqn 1
The average atomic weight of Magnesium is 24.305 amu. Thus,
23.99(0.79) + 24.99(x) + 25.99(y) = 24.305 --> eqn 2
Rearranging eqn 1: x = 1 - 0.79 - y = 0.21 - y. Substituting this to the second equation,
23.99(0.79) + 24.99(0.21 - y) + 25.99(y) = 24.305
Solving for y,
y = 0.105 or 10.5%
Thus, the percent abundance of 26 mg is y which is 10.5%.
