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.

[Q]: Problem: a mixture of 70 litres of milk and water contains 10 % water. how many litres of water should be added to the mixture so that the mixture contains 12 1 / 2 % water ?
Options: a. 2, b. 8, c. 6, d. 3, e. 4
[A]: a


[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
[A]: e


[Q]: Problem: a movie buff owns movies on dvd and on blu - ray in a ratio of 7: 2. if she buys 4 more blu - ray movies, that ratio would change to 13: 4. if she owns movies on no other medium, what was the original number of movies in her library before the extra purchase ?
Options: a. 22, b. 28, c. 77, d. 99, e. 234
[A]:
e