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.

[EX Q]: Problem: x does a work in 20 days. y does the same work in 40 days. in how many days they together will do the same work ?
Options: a. 10, b. 12, c. 20, d. 30, e. 13.33
[EX A]: e

[EX Q]: Problem: if 80 percent of a class answered the first question on a certain test correctly, 55 percent answered the second question on the test correctly, and 20 percent answered neither of the questions correctly, what percent answered both correctly ?
Options: a. 10 %, b. ( 8 ) 20 %, c. 30 %, d. 50 %, e. 55 %
[EX A]: e

[EX Q]: Problem: a case of 12 rolls of paper towels sells for $ 9. the cost of one roll sold individually is $ 1. what is the percent w of savings per roll for the 12 - roll package over the cost of 12 rolls purchased individually ?
Options: a. 9 %, b. 11 %, c. 15 %, d. 25 %, e. 90 %
[EX A]:
d