Correct! And, as it turned out, today was part of that 80%
Solution:
Say you see her working on a Friday. The possibilities for her two consecutive days off are:
MT, TW, WTh, SaSu, SuM
We want to know the probability that she'll be working the next day, Saturday. Of her five possible schedules, only one has her off on Saturday. The other four out of five schedules have her working Saturday, so the answer is 4 out 5.