Let $x_1,x_2,x_3$ and $x_4$ be the decisions variables for the amounts of a raw material to source from four different suppliers respectively. If we desire that a minimum of $20\%$ of the total be provided by supplier $2,$ the required constraint is

$-0\mathrm{.}2x_1+0\mathrm{.}8x_2-0\mathrm{.}2x_3-0\mathrm{.}2x_{4}0$

$x_{2}0\mathrm{.}20$

$-0\mathrm{.}2x_1+0\mathrm{.}8x_2-0\mathrm{.}2x_3-0\mathrm{.}2x_{4}0$

$0\mathrm{.}8x_1-0\mathrm{.}2x_2+0\mathrm{.}8x_3+0\mathrm{.}8x_{4}0$