if we breathe in 21% of oxygen what percentage do we breathe out

4 %

21% oxygen is the amount of osygen in air that a person inhale in a single breath.
There is a respiration processs inside the body which converts the food particle into energy ( calories)
The resultant factor is the Carbon Dioxide that comes out as the gas .
If someone could correct me

C6H1206 -----decomposed inside body --into -- C02 +H20 . Water wavors plus carbon dioxide is emitted

well you only breath out the unwanted gases... and oxygen is vital for our survival so the answer is zero %... We use everything and so there is no oxygen to breath out

We exhale approximately 14% oxygen and nearly 6% carbon dioxide, their for we only use 6% oxygen. This may vary depending on age, sex, size, and most importantly activity level.

Edit;
If we used all of the 21% Rescue breaths in CPR would be pointless.
The body only uses a small amount of oxygen to met its requirements, the rest is exhaled, with carbon dioxide.

The average is 16%. This can vary depending on age, sex, co-morbidity, altitude.