Abacterial gene 900 nucleotides long can code at most for a polypeptide of how many amino acids?

300 amino acids. Since an amino acid can only be coded by a codon, and a codon is comprised of three nucleotide bases, then 900/3 will give us the number of amino acid in a polypeptide that can be coded by the gene.  

oxygen = 21% of the atmosphere

nitrogen = 78%

carbon dioxide = 0.1%

argon =0.934%