# Permutations without Repetition

• October 29th 2009, 06:09 AM
creatively12
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 ?

• October 29th 2009, 06:14 AM
creatively12
I just tried this again, and i got the answer 64, am I correct ???
• October 29th 2009, 06:18 AM
Plato
Quote:

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 $\sum\limits_{k = 1}^4 {_4 \mathcal{P}_k }$.
• October 29th 2009, 06:29 AM
creatively12