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: the speed of the boat in still water in 12 kmph. it can travel downstream through 48 kms in 3 hrs. in what time would it cover the same distance upstream ?
Options: a. 8 hours, b. 6 hours, c. 4 hours, d. 5 hours, e. 6 hours
[EX A]: e

[EX Q]: Problem: a theater box office sold an average ( arithmetic mean ) of 63 tickets per staff member to a particular movie. among the daytime staff, the average number sold per member was 78, and among the evening staff, the average number sold was 60. if there are no other employees, what was the ratio of the number of daytime staff members to the number of evening staff members ?
Options: a. 2: 3, b. 1: 4, c. 1: 5, d. 6: 11, e. 4: 7
[EX A]: c

[EX Q]: Problem: a cistern can be filled by a tap in 2 hours while it can be emptied by another tap in 4 hours. if both the taps are opened simultaneously then after how much time will the cistern get filled ?
Options: a. 2 hr, b. 5 hr, c. 7 hr, d. 1 hr, e. 4 hr
[EX A]:
e