With 20 amino acids to choose from how many different combinations are there for a protein thats three amino acids long

In order to determine the number of proteins that may be formed, we will use the concept of permutations. Permutations are basically the ordered arrangement of items where the order of the items matters. These are applicable in this case because the order of the amino acids does matter in determining the protein that is made. The formula is:

nPr = n! / (n-r)!; where n is the number of elements and r is the number of elements required in each set. Thus,

20P3 = 20!/(20 - 3)!
= 6,340

The amino acids may be combined in 6,340 ways.