The principal naturally occurring isotope of magnesium has an atomic mass of 23.98504 u and a 79.0% abundance.

The principal naturally occurring isotope of magnesium has an atomic mass of 23.98504 u and a 79.0% abundance. Two other naturally occurring isotopes have atomic masses of 24.98584 u and 25.98259 u, respectively. Determine the percent abundances each of the three naturally occurring isotopes of magnesium.

Can someone explain how to get the two percent abundance of the isotopes

79.0% + %Mg-25 + %Mg-26 = 100% Equation (1)
(79.0% * 23.98504 + %Mg-25 * 24.98584 + %Mg-26 * 25.98259) / 100% = 24.3051 Equation (2)

Rearranging equations 1 and 2 give

%Mg-25 + %Mg-26 = 21% Equation (1a)
1894.8181%6 + %Mg-25 * 24.98584 + %Mg-26 * 25.98259 = 2430.51% (Equation 2a)

%Mg-25 + %Mg-26 = 21% Equation (1b)
%Mg-25 * 24.98584 + %Mg-26 * 25.98259 = 535.6919% (Equation 2b)

%Mg-25 * 24.98584 + %Mg-26 * 24.98584 = 524.70264% Equation (1c)
%Mg-25 * 24.98584 + %Mg-26 * 25.98259 = 535.6919% (Equation 2c)
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%Mg-26 * 0.99675 = -10.98926%

%Mg-26 = 11.0%

Substituting the above results into equation 1c gives

%Mg-25 * 24.98584 + 11.0% * 24.98584 = 524.70264%

%Mg-25 * 24.98584 + 274.8% = 524.70264%

%Mg-25 * 24.98584 = 249.90264

%Mg-25 = 10.0%