How many different tripeptides can exist,each containing one residue of glycine,one of L-threonine,and one of L-arginine?

Question

Quizplus · Accepted Answer

For the tripeptide to contain one residue of glycine, one of L-threonine, and one of L-arginine, we must consider all possible arrangements of these three amino acids. 
These are: Gly-Thr-Arg, Thr-Gly-Arg, Thr-Arg-Gly, Gly-Arg-Thr, Arg-Gly-Thr, and Arg-Thr-Gly. 
Therefore, there are 6 different tripeptides that can exist, making the answer (C).

How Many Different Tripeptides Can Exist,each Containing One Residue of Glycine,one