About the revision vedio-Inventory

Hello, How can it be certain that the total cost will be the minimum at the level at which we first get discount? eg: how can we know at quantity of 200, we can get the lowest cost instead of other between 200~499, like 300? although it is obvious that the holding cost will higher at 300 than 200, but the ordering cost will much lower? how can we decide?

Looking for tutor’s advice