Question 920305
Use variables, c atomic mass units for Carbon, and h atomic mass units for Hydrogen.


{{{system(2c+6h=30.07,3c+8h=44.097)}}}


Start with elimination method to eliminate c to first solve for h.  Multiply the ethane equation by 3 and the propane equation by 2, then subtract the equations.


{{{system(6c+18h=90.21,6c+16h=88.194)}}}


{{{2h=2.016}}}
{{{highlight(h=1.008)}}}


Use any method you want to find the amu of carbon.  You already know to expect something just a small fraction slightly larger than 12.