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
Direct variation can be written as a proportion. Therefore, we can write:

Cross multiply to get:
4y=40
Divide by 4 on both sides to get that
y=10.
:)