Thread: How many bit strings of length 27 are there...?

How many bit strings of length 27 are there such that:

a. the bit string corresponding to the last twelve positions contain exactly seven 0

b. the bit string has at least fifteen 0 and at least ten 1, also must have the bit string corresponding to the first nine positions contains six 1and the bit string corresponding to the last twelve positions contain a maximum of ten 0.

c. the bit string corresponding to the first ten positions contain exactly eight 1 and a bit string corresponding to the last fifteen positions contains (or does not contain) the string as a sub-string 1011101


Help me please!!!

Hi,

I will attempt a.

For the last 12 positions there are ways to pick the 0's, once the 0's are fixed, the 1's fall naturally into position.

Then for the first 15 positions there are 2 choices for each positions, either a 1 or 0, thus there are choices for the 1st 15 positions.

Thus in total we have bit strings.

I would really like to know if this is correct though, could someone confirm? Thanks.