Let me start out by staying math has never been my strongest area and so please don't roast me lol.

I was recently in Colorado with a friend and we visited pikes peak. In the visitor center there's a cool plaque that shows the difference in oxygen levels at different elevation.

These numbers aren't exact but from what I remember it said something like-

Sea level- 100 percent available oxygen.

6000 ft (Colorado springs) 80 percent available oxygen

Pikes peak, 14,500 ft - 60 percent available oxygen.

So I said something like "wow, so we have 20 percent less oxygen than we did at the bottom of the mountain"

My friend laughed at me and kinda embarrassed me in front of everyone and was like "That's not how percentages work at all"

So can someone please actually kindly explain to me why and how I'm wrong and what the difference actually is?