If a 12 furnaces consume 14 tones of coal. How much coal would be needed for 18 furnaces at the same rate?

one furnace uses 14/12, so 18 furnaces use 14/12 times 18 coal. this is 21

12/14=6/7   so 12+6=18. and 14+7=21.  21 tons of coal would be needed for 18 furnaces.