Chapter 4 - Amino Acids - Exercises - Page 95: 6

$120$

$N$ = $5!$ = $120$ The first residue can be one of the five, the second residue can be one of the four, the third residue can be one of the three, the fourth residue can be one of the two, and the fifth residue is the last one remaining. Thus, we know the value is equal to five factorial.