# Math Help - Permutations without Repetition

1. ## Permutations without Repetition

How many different numbers can be formed by taking one, two, three, and four digits from the digits 1, 2, 7, 8, if repetition are not allowed ?

Using the notation $_N\mathcal{P}_k=\frac{N!}{(N-k)!}$
the answer is $\sum\limits_{k = 1}^4 {_4 \mathcal{P}_k }$.