Hi Liliana,

This s a strange question. Suppose the four letters are A, E, I and O. Any of the three different letters, chosen in any order is a permutation. For example

AIO
EAI
AIE
IEO
OAE

are five different, three letter permutations of the letters A, E, I and O. Notice that the second and third in my list are composed of the same letters but since they are in different orders they are different permutations.

Permutations are sometimes compared to combinations. A three letter combination of the letters A, E, I and O is any subset of the set of letters A, E, I and O. Here order is not important. So in the example of five, three letter permutations above there are only four combinations since EAI and AIE are the same combination.

I hope this helps,
Penny