[tex]Brazilian-5$\ per\ kg\\\\
Turkish-\ 8$\ per\ kg\\\\
x-amount\ of\ Brazilian\ coffee\\
y-amount\ of\ Turkish\ coffee\\\\
x*5+(1-x)*8=7\\
5x+8-8x=7\\
-3x=-1\\
x=1/3\\
y=2/3\\\\
To\ make\ 300kg\ we\ need:\ (from\ proportion)\\
\frac{2}{3}-1kg\ \ \ to\ make\ 1kg\ we\ need\ \frac{2}{3}\ Turkish\ coffee\\
a\ \ \ -300kg\ \ \ to \ make\ 300kg\ we\ need\ a\ Turkish\ coffee\\
300*\frac{2}{3}=a\\
a=200kg\ \ He\ need\ 200kg\ Turkish\ coffee.[/tex]