The system is something like this (you get fractional parts):
[tex]x+y=1 \\ 9.2x+11y=10.28 \\ \\ 11x+11y=11 \\ 9.2x+11y=10.28 \\ 1.8x=.72 \\ x=\frac{\frac{18}{25}}{\frac{9}{5}}=\frac{18}{25}*\frac{5}{9}=\frac{2}{5}[/tex]
You get 2 parts of $9.20 coffee and 3 parts of $11 coffee in the mixture.