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!!! (Worried)