Applied to a real situation I’ve been through :

• my pool 4.5m wide, 9m long and 1.5m deep, the current level of salt is 2.5g/l and a bag of salt weight 20kg. How many bags of salt do I need to bring the level to 3g/l ?
• OR: my pool 14’9" wide, 29’6" long and 5’ deep, the current level of salt is 2500ppm and a bag of salt weights 40lbs. How many bags of salt do I need to bring the level to 3000ppm ?

The answer to one is 1.5 bag, the answer to the other one is “fuck that, I’m getting 8 bags at the store and it should be good enough”