Answer
The mass of the third magnesium isotope is equal to 26 $u$.
Work Step by Step
Atomic weight for magnesium : 24.305 $u$ (Periodic table)
$(Mass^{24}Mg)(Abundance^{24}Mg)$ + $(Mass^{25}Mg)(Abundance^{25}Mg) + (Mass^{x}Mg)(Abundance^{x}Mg) = Atomic$ $weight$
$(23.985)(0.7899) + (24.986)(0.1000) + (Mass)(0.1101) = 24.305$
$ (Mass)(0.1101) = 24.305 - (23.985)(0.7899) - (24.986)(0.1000) $
$Mass = \frac{24.305 - (23.985)(0.7899) - (24.986)(0.1000) }{0.1101}$
$Mass = 25.982\ u$
$Mass \approx 26\ u$
