I mentioned at the end of class that some of the exercises in phonology were a little more challenging than usual. Here's a tip: think of phonemes as Superman (or most superheros who disguise their identity). You never see Superman and Clark Kent in the same place at the same time. Superman only appears in a specific environment (i.e., when there is danger or a crime is being committed). He is CK the rest of the time. This is the equivalent of being in complementary distribution. Allophones are the same: they never appear in the same environment at the same time, thus we know that they are a single entity (a phoneme). In fact, the analogy carries further: we think of the default value of Superman/CK as CK, right? Because he is CK most of the time, and only becomes Superman in a specific environment. It is exactly the same with allophones of a single phoneme. The default value occurs in the widest variety of environments, and in some specific environments, a change occurs (to the other allophone).

I hope this helps.