Frequency of Amino Acids

Im currently starting a biostats summer course and Im still confused about how to calculate probability.

My Question is

If the overall genome frequency of cytosine (A) and guanine (T) is A=0.2 and T=0.2, at what frequency would you expect to find the sequence AT, assuming random order?

If you observed N ATs out of a genome of length L, how might you determine whether the observed N is significantly underrepresented relative to expectation?