# 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 ?

2. I just tried this again, and i got the answer 64, am I correct ???

3. Originally Posted by creatively12
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 $\sum\limits_{k = 1}^4 {_4 \mathcal{P}_k }$.

so my answer is correct right ???