પ્રમાણ$\%$  =             $ x\% $        $ (100-x)\%$  

સરેરાશ પરમાણુભાર  $ = {\text{ }}\frac{{{\text{x}} \times {\text{12}}}}{{{\text{100}}}} + \frac{{(100 - x) \times 13}}{{100}}$

$12.011\,\,\, = \,\,\frac{{12x}}{{100}} + \frac{{1300 - 13x}}{{100}}\,\,\, \Rightarrow \,\,\,12.011\,\,\, = \,\,\frac{{12x + 1300 - 13x}}{{100}}\,\, = \,\,\frac{{ - x + 1300}}{{100}}$

$ - x + 1300 = 1201.1\,\,\, \Rightarrow \,\,\, - x = 1201.1 - 1300\,\,\, \Rightarrow \,\, - x =  - 98.9\,\,\,\therefore \,\,x = 98.9\% $

$\therefore \,\,{C^{12}}\,\, = \,\,98.9\% \,,\,\,\,\,{C^{13}}\,\, = \,\,1.1\% $