Please explain why I'm right or why I am wrong.

Okay so let us say we have a patient on 2L of oxygen. Coming from a type E oxygen bottle with about 100% pure O2.

Now normal oxygen in the air is about 20%, so if we put them on 10L of air it should be the same as 2L of the pure stuff.

Why is this important?
Because there is soon to be massive hospital overcrowding and up to 20% of those infected need oxygen for 2 weeks or they die. So this may be a simple solution. Also yes I realize 2L is fine for a nasal cannula, but 10L requires a rebreather mask.