[tex]x-chocolate\ worth\ 1,50\$ \ per\ pound\\
y-chocolate\ worth\ 0,9\$\ per\ pound\\
\\\\
\left \{ {{x+y=30} \atop {30*1,10=x*1,50+y*0,9}} \right.\\\\ \left \{ {{x=30-y} \atop {33=1,5x+0,9y}} \right. \\\\ Using \ substitution\ method:\\\\33=1,5(30-y)+0,9y\\\\
33=45-1,5y+0,9y\\\\ -12=-0,6y\ |:-0,6\\\\
y=20\\
x=10[/tex]