Biochemistry 4th Edition by Reginald Garrett, Charles Grisham

Exercise 9

Combinatorial libraries: calculating the number of sequence possibilities for oligonucleotides and peptides
Combinatorial chemistry can be used to synthesize polymers such as oligopeptides or oligonucleotides. The number of sequence possibilities for a polymer is given by xy, where x is the number of different monomer types (for example, 20 different amino acids in a protein or 4 different nucleotides in a nucleic acid) and y is the number of monomers in the oligomers.
a. Calculate the number of sequence possibilities for RNA oligomers 15 nucleotides long.
b. Calculate the number of amino acid sequence possibilities for pentapeptides.