Consider the coal processing and delivery homework quesiton in a smaller scale: we have two mines $($i $=1,2)$ and two power plants $($j $=$ A$,$ B$).$ Let Xij $=$ tons of coal produced at Mine i and delivered to Plant j$,$ where i $=1,2$ and j $=$ A$,$ B$.$ Mine $1\text{'}$s capacity is $13,000$ tons and Mine $2\text{'}$s capacity is$15,000$ tons, and the minimum demand of Plants A and B are $11,000$ tons and $14,000$ tons, respectively. $($Tips: Do initial comparison between total supply and total demand to decide the sign to use for the supply and demand constraints.$)$