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.

[EX Q]: Problem: how many 4 - digit numbers that do not contain the digits 2, 3, 4, 5, 6, or 7 are there ?
Options: a. 168, b. 176, c. 184, d. 192, e. 200
[EX A]: d

[EX Q]: Problem: if 32.5 % of the 880 students at a certain college are enrolled in biology classes, how many students at the college are not enrolled in a biology class ?
Options: a. 110, b. 330, c. 550, d. 594, e. 880
[EX A]: d

[EX Q]: Problem: a cistern can be filled by a tap in 6 hours while it can be emptied by another tap in 12 hours. if both the taps are opened simultaneously, then after how much time will the cistern get filled ?
Options: a. 5.2 hrs, b. 2.9 hrs, c. 12 hrs, d. 7.2 hrs, e. 1.4 hrs
[EX A]:
c