The portion of average mass of C-12 is = the percentage
abundance of C-12 x atomic mass of C-12
                                                      Â
= (76.3/100) x 12
                                                      Â
= 9.156
The portion of average mass of C-14 is = the percentage
abundance of C-14 x atomic mass of C-14
                                                      Â
= (23.7/100) x 14
                                                      Â
= 3.318
So total mass is = The portion of average mass of C-12 + The
portion of average mass of C-14
                       Â
= 9.156 + 3.318
                       Â
= 12.474
Therefore the average atomic mass is 12.474 amu