A DNA code is a sequence of the symbols A,G,C,T (called bases).

a) How many possible such sequences are there of length 20 containing 5 of each base?

b) How many such sequences (containing 5 of each base) are there if we can't distinguish a sequence from its mirror image?

c) What is the number of DNA sequences of length 10 which don't contain two consecutive bases which are the same (this means: no AA,GG,CC,TT in the sequence)?

I've been trying these 3 problems for a while and any suggestions on how to go about doing any of them would be greatly appreciated.

Thanks for your help in advance!