My stat window in game said i had 51% tenacity

i had merc treads i did not have the tenacity masteries and i had the elixir of iron cuz peeps wanted to see a giant cho and we were enough ahead for it. So is the elixir of iron separate from any other unique passives does it let you really get that much tenacity. Cuz i got ulted by amumu and then ww back to back with the elixir up and it seemed like it worked and it felt nice to shrug em off but i want to make sure it wasn't in my head. So if you went 9-21 irelia got merc treads and the elixir and were in a 5v5 fight what would your tenacity be????