Question

One liter of air contains about 0.21 L of oxygen. When filled, the human lungs hold about 6.0 L of air. How much oxygen is in the lungs when they are filled with air? (Round off to the nearest hundredths place.)

Answer 1

Since we are told that 1L of air contains 0.21L of oxygen, you can use the conversion (0.21L O₂)/(1L air).  That means that you can just multiply 6.0L by 0.21L to get 1.26L of O₂.
that means that the lungs can hold about 1.26L of oxygen.
I hope this helps.  Let me know if anything is unclear.