# How do you calculate the average atomic mass of carbon if 98.90% of the atoms are C-12 (12.000000 and 1.100% are C-13 atoms (13.003354 amu)?

Average atomic mass of carbon $= \left(0.989 \cdot 12\right) + \left(0.011 \cdot 13.003354\right) = 12.011036894$