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## Homework Statement

In a binary communication system all bit bytes are possible, and equally likely. Any bit is equallly likely to be a 1 or 0.

1- what is the probability of oberving a byte with exactly three 1's?

2-if a byte is observed with exactly three i's, what is the probability the first bit is a 1?

## Homework Equations

combination formula

**nCp**

## The Attempt at a Solution

1- I did:

**P(exactly three 1's)= C(3,8)/ 2^8**

2-

Here I am confused:

I did

**P( three 1's and 1 is first)=C(2,7)/(2^8)**

OR

**P(three 1's and 1 is first)=C(3,7)/(2^8)**

I cannot see which one is right.

Can someone tell me what I did right and wrong?

Thank you

B