Answer:

    (a)    (b)    (c)
    1-20  21-40  41-60  a+b  all
A    6      3      6     12   15
C    4      7      7     11   18
G    6      6      3      9   15
T    4      4      4      8   12

18! 15! 15! 12! 3!     7! 6! 4! 3! 3!   12! 11! 9! 8! 3!
------------------  v. -------------- . ----------------
        63!                 23!                43!

take neg logs:

     null      v.   GC-rich "red"
    -----           -------------
    201.0 (63)        121.5  (43)
   - 36.4 (18)       + 51.6  (23)
   - 27.9 (15)       - 20.0  (12)
   - 27.9 (15)       - 17.5  (11)
   - 20.0 (12)       - 12.8   (9)
   -  1.8  (3)       - 10.6   (8)
   ------            -  8.5   (7)
     87.0 nits *     -  6.6   (6)
                     -  3.2   (4)
                     -  1.8   (3)
                     -  1.8   (3)
                     -  1.8   (3)
                    -------
                       88.5 nits

Answer: the null wins by a small margin (odds about 4:1)

Please [check]!

(And note that if the supposed cut points (20+ & 40+) were not "common knowledge" but had to be stated in the hypothesis, that would add about 8 nits more to the "red" hypothesis.)