The main function is to help them see with all their eyes.
        
                    
             
        
        
        
 12% NaCl is the answer I came to... Have a GREAT day!!! :)
        
             
        
        
        
First step in answering the question is to establish a balanced chemical reaction equation. More specifically, a combustion chemical equation. 
CH4 + 2O2 ---> CO2 + 2H20
Then using dimension analysis: 
