As the wiki explains multiplicative satcking:
When effects stack multiplicatively, the cumulative effect can be calculated by multiplying the effects together. Note that the game interface rounds champion statistics, so what is shown in-game may be slightly different than what is calculated.
It is important to understand that most champion statistics that stack multiplicatively are displayed as the opposite of the actual numbers that are multiplied. For example, 10% damage reduction should instead be understood as 90% damage taken, and 25% damage reduction should instead be understood as 75% damage.
Example: If your champion has armor granting 10% damage reduction and an item granting 10% damage reduction, you multiply the damage factors—0.9 and 0.9—to get 0.81, which means your champion has 19% damage reduction (against physical damage).
It even included this handy chart:
https://i.imgur.com/twwrTN1.png[/img]
So what this means, is that if an item grants 30% tenacity, think of it as 0.7 as 30% tenacity means CC lasts 70% as long. Multiple all effects in this manner to see what your total will be.