Each building adds to the base production, not to each other.
So roughly, a basic province has the potential to produce 3,000 tons of resource per day at 100% morale.
Lower province morale results in lower production.
If you have a building that will increase production by 50%, that is 50% of 3000, so 1500 additional.
Another building could do 50%, for another 1500.
3000 + 1500 + 1500 = 6000
So double the initial production.
