Carbon occurs in nature as a mixture of ¹²C and ¹³C. The average atomic mass of carbon is 12.011. what is the % of abundance of ¹²C in nature?

Let the % of abundance of ¹²C = x
% of abundance of ¹³C = 100 - x
$\because \frac{12\mathrm{x}}{100}+\frac{13(100-\mathrm{x})}{100}=12.011$
12x + 1300 - 13x = 1201.1 → x - 98.9
Thus, % of abundance of ¹²C is 98.9 %.