How can there be 64 codon combinations but only 20 possible amino acids?

Hint: Amino acids are small organic molecules that are also known as the building block of proteins as the proteins are formed by the joining of amino acids to each other.

Complete answer:
1)A codon is known as the sequence where three nucleotides are arranged together and this sequence of nucleotides codes for a particular amino acid.
2)Now this sequence may contain the same or different nucleotides in a sequence and 64 such combinations can be made by arranging the nucleotides differently but this 64 codon combination will not be coding for different amino acids and only 20 amino acids are possible which means that 1 or more codon combination can be indicated by a single amino acid. This is because one of the features of genetic codes is degeneracy.
3)Out of 64 codon combinations, only 61 codons code for amino acids, the rest 3 codons are 'stop' codons and they do not code for any amino acids. The stop codons are given by sequences UAA, UAG, and UGA.

Note: The degeneracy of the genetic code can be illustrated by the following examples, the amino acid leucine can be described by many sequences that are UUA, UUG, CUU, CUC, CUA, CUG. The amino acid Valine can be described by sequences GUU, GUC, GUA, GUG. Amino acid methionine can be described by only one sequence is AUG, which is also known as the start codon.