If there are 4 nucleotides instead of 3 in codon, how many amino acids may be formed ?
Formula for calculating no. of codons = (4)n

n = no. of nucleotides that constitute codon.
e.g. if n = 2; no. of codons = (4)2 = 16
if n = 3; no. of codons = (4)3 = 64
if n = 4; no. of codons = (4)4 = 256
so on
16
21
256
64