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: 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
e

Problem:. 005 / ? =. 01
Options: a.. 5, b.. 05, c.. 005, d.. 0005, e. none of them
a

Problem: ( 212 + 222 + 232 +... + 302 ) = ?
Options: a. 385, b. 2485, c. 2570, d. 3255, e. none
c