No idea where you got your 33.(3)% from, but 16.(6)% is the right answer. You have 2 bins for cp_a and 3 bins for cp_c, this means that cp_c has 2x3 = 6 bins. Filling one of those bins will mean you have 1/6 = 16.(6)% coverage.
No idea where you got your 33.(3)% from, but 16.(6)% is the right answer. You have 2 bins for cp_a and 3 bins for cp_c, this means that cp_c has 2x3 = 6 bins. Filling one of those bins will mean you have 1/6 = 16.(6)% coverage.