Given the task definition and input, reply with output. Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

Problem: maths, physics and chemistry books are stored on a library shelf that can accommodate 25 books. currently, 20 % of the shelf spots remain empty. there are twice as many maths books as physics books and the number of physics books is 4 greater than that of chemistry books. among all the books, 12 books are soft cover and the remaining are hard - cover. if there are a total of 7 hard - cover books among the maths and physics books. what is the probability q, that a book selected at random is either a hard cover book or a chemistry book ?
Options: a. 1 / 10, b. 3 / 20, c. 1 / 5, d. 1 / 4, e. 9 / 20

e