# 1 combination problem

• Oct 20th 2008, 02:56 AM
Narek
1 combination problem
Thank you in advance. please if possible, explain for me, so i can solve other problems by myself (Rofl)
• Oct 20th 2008, 03:05 AM
ddt
Quote:

Originally Posted by Narek
Thank you in advance. please if possible, explain for me, so i can solve other problems by myself (Rofl)

A 'bit' is a 0 or a 1 (as in computers)
A 'string' is just another word for a sequence.
So a 'bit string of length 10' is a sequence of 10 bits. Example: 0010110100
• Oct 20th 2008, 04:47 AM
Narek
Quote:

Originally Posted by ddt
A 'bit' is a 0 or a 1 (as in computers)
A 'string' is just another word for a sequence.
So a 'bit string of length 10' is a sequence of 10 bits. Example: 0010110100

uhum. i understand. but this is not answer to my question (Doh)
• Oct 20th 2008, 05:42 AM
ddt
Quote:

Originally Posted by Narek
uhum. i understand. but this is not answer to my question (Doh)

OK, but it wasn't clear what part of the problem you didn't understand.

So, taking the first of your questions:

"How many bit strings of length 10 have exactly three 0s?"

is equivalent to asking: in how many ways can I make a set of 3 positions out of 10? (the positions being the positions in the bit string where the 0s occur - the other 7 positions then have 1s).

Hint: Combinations; Binomium of Newton.
• Oct 20th 2008, 05:51 AM
Narek
i tried to answer them. is these true?

a) C(10,2)

b) C(10,5)

c) C(10,7) + C(10,8) + C(10,9) + C(10,10)

d) C(10,3) + C(10,4) + C(10,5) + C(10,6) + C(10,7) + C(10,8) + C(10,9) + C(10,10)

Thank you (Rofl)
• Oct 20th 2008, 06:20 AM
ddt
Quote:

Originally Posted by Narek
i tried to answer them. is these true?

a) C(10,2)

b) C(10,5)

c) C(10,7) + C(10,8) + C(10,9) + C(10,10)

d) C(10,3) + C(10,4) + C(10,5) + C(10,6) + C(10,7) + C(10,8) + C(10,9) + C(10,10)

Thank you (Rofl)

(a) is C(10, 3) - I guess that's a typo? For the rest, it's correct yes.