Read the following statements carefully then answer the question by filling in the blank spaces.                                                                                                                                

The results shown were obtained by a form 1 student in an experiment to determine the percentage of oxygen in the air.                                                                                        
Initial volume of air= 100cm3
Vol. of air after expt= 79cm3.
Calculate the percentage of oxygen in the air by volume.                
Vol of air used=cm3                                                  
%age of oxygen=%