Answer is: the average atomic mass of carbon is 12.011 amu.

m(¹²C) = 12.00 amu; the average atomic mass of carbon-12.  

m(¹³C) = 13.003354 amu; the average atomic mass of carbon-13.

ω(¹²C) = 98.90% ÷ 100% = 0.989; fractional abudance of carbon-12.

ω(¹³C) = 1.100% ÷ 100% = 0.011; fractional abudance of carbon-13.

m(C) = m(¹²C) · ω(¹²C) + m(¹³C) · ω(¹³C).  

m(C) = 12.0 amu · 0.989 + 13.003354 amu · 0.0110.  

m(C) = 12.011 amu; average atomic mass of carbon.

The unified atomic mass unit (amu) is a standard unit of atom