Relative Abundance of Isotopes

Naturally occurring magnesium has 3 isotopes : Mg-24, Mg-25, Mg-26. The relative abundance of the Mg-26 isotope is 10%. What is the relative abundance of the Mg-26 isotope?
(Relative atomic mass : 24.3)


Let y% be the relative abundance of Mg-26.
Then, the relative abundance of Mg-26 = (100 - 10 - y)% = (90 - y)%

Relative atomic mass of Mg:
24(90 - y)% + 25(10%) + 26(y%) = 24.3
21.6 - 0.24y + 2.5 + 0.26y = 24.3
0.02y = 0.2
y = 10

The relative abundance of the Mg-26 isotope = 10%

The relative abundance of the Mg-26 isotope is 10%.

Let x be the relative abundance of 26 Mg isotope,then
25x10%+24(1-10%-x)+26x = 24.3
2.5 + 21.6x - 24x +26x = 24.3
23.6x = 21.8
x = 92.4%(corr to 3 sig figs)