Teacher: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.
Teacher: Now, understand the problem? Solve this instance: Problem: a can do a piece of work in 80 days. he works at it for 10 days b alone finishes the remaining work in 42 days. in how much time will a and b working together, finish the work ?
Options: a. 25, b. 30, c. 20, d. 32, e. none of them
Student:
b