Suppose there's a crevice or deep hole in a river and all the minerals in the table above are bouncing along the stream's bottom. Heavy minerals will be deposited in the law areas but less dense material will be carried away by the flowing water. What materials would likely be found concentrated along with magnetite in placer deposits? Placer materials settle in holes or crevices at the bottom of the stream because of their higher density. Why should you care about this? Because many precious ores and metals are concentrated with magnetite along the irregular bottoms of streams- The California Rush was impelled by the discovery of placer gold at Sutter's Mill on the American River associated with 'black sand" (magnetite) deposits.
Which materials might be smelted in furnaces that reach a temperature of only 1100°C? Why should you care about this? Because early furnaces could attain temperatures near 1100oC and that only rarely, some ores could be dumped in a furnace but they'd just get hot and wouldn't melt. They might keep toes warm during a cold night but that was just about all they'd be good for.
If a flux that lowers the temperature of each of the above substances by 3003C, which additional materials can be melted in a furnace whose maximum temperature is Only 1100°C? Why should you care about this? Because it requires much less energy to melt something at a lower temperature. Hence people look for a substance (flux) that will lower the melting temperature of a desired substance.
How can you determine which substances would melt with such a flux? Take a hypothetical example. Suppose your table contained the mineral cuprite (a copper oxide) that has a melting temperature of 1235 °C. Obviously it wouldn't melt in a furnace whose temperature could only reach 1100°C. However, a flux which lowered the temperature of melting by 300T from 1235oC - 300oC a 935°C would melt cuprite in such furnace.