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.
One example: Problem: a multiple choice test consists of 4 questions, and each question has 5 answer choices. in how many r ways can the test be completed if every question is unanswered ?
Options: a. 24, b. 120, c. 625, d. 720, e. 1024
Solution is here: c
Explanation: 5 choices for each of the 4 questions, thus total r of 5 * 5 * 5 * 5 = 5 ^ 4 = 625 ways to answer all of them. Hence, the answer is c.

Now, solve this: Problem: a man can do a piece of work in 5 days, but with the help of his son, he can finish it in 4 days. in what time can the son do it alone ?
Options: a. 5, b. 5 1 / 2, c. 7 1 / 2, d. 6, e. 20
Solution:
e