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: two pipes can fill a tank in 12 minutes and 20 minutes respectively. both pipes are opened together and after some time the first pipe is closed and the tank is full in totally 10 minutes. for how many minutes was first pipe open ?
Options: a. 8 minutes, b. 6 minutes, c. 7 minutes, d. 10 minutes, e. 9 minutes
A:
b