1. n choose k question

Question: How many n-digit binary (0,1) sequences contain exactly k 1s?

Could someone explain to me why the answer is n choose k? I don't understand it.

Thanks!

2. Originally Posted by absvalue
Question: How many n-digit binary (0,1) sequences contain exactly k 1s?
Could someone explain to me why the answer is n choose k? I don't understand it.
There is a string of n symbols, either a zero or a one.
So you have k places to put a one. So choose k from n.
Put zeros everywhere else.

3. Originally Posted by Plato