There are 20 C3 possible ways to choose 3 books to read. There are 5 C3 possible ways to choose 3 mysteries to read. The probability that Mariah will not choose all mysteries can be expressed as 1-5C3/20C3. (These 3 Choices Are True)
There are 1 C3 possible ways to choose 3 books that are not all mysteries. The probability that Mariah will choose 3 mysteries can be expressed as 1/5C3. (These 2 Choices Are False) Hope This Helps