Let's start by setting up some equations to represent the information given in the problem.
Let's assume that Mitchell read x sports books last year. According to the problem, he read 5 mysteries for every 2 sports books, so he must have read (5/2)x mysteries.
We also know that he read 12 more mysteries than sports books, so:
(5/2)x - x = 12
Simplifying this equation, we get:
(3/2)x = 12
Multiplying both sides by 2/3, we get:
x = 8
So Mitchell read 8 sports books last year. To find out how many mysteries he read, we can use the equation we derived earlier:
(5/2)x = (5/2)(8) = 20
Therefore, Mitchell read 20 mysteries last year.