Suppose we want to choose 3 letters without replacement from the 4 letters a, b, c and d, how many ways can this be done if the choice matters or does not matter

it is the number of permutations that are possible when arranging k k ... You must choose two, without replacement, where order matters ... Here are lots of different names for this number: 5P2=5⋅ 4=5⋅4⋅3!3! ... How many ways are there to choose two letters from five letters (without ... Then, n C k=nPkk!