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.
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Question: Problem: buses a and b start from a common bus stop x. bus a begins to travel in a straight line away from bus b at a constant rate of 30 miles per hour. one hour later, bus b begins to travel in a straight line in the exact opposite direction at a constant rate of 50 miles per hour. if both buses travel indefinitely, what is the positive difference, in minutes, between the amount of time it takes bus b to cover the exact distance that bus a has covered and the amount of time it takes bus b to cover twice the distance that bus a has covered ?
Options: a. 36, b. 84, c. 132, d. 144, e. 180

Answer: b


Question: Problem: if a sum of money doubles itself in 20 years at simple interest, the ratepercent per annum is
Options: a. 12, b. 12.5, c. 13, d. 5, e. 14

Answer: d


Question: Problem: if x = 3 ^ 9 and x ^ x = 3 ^ k, what is k ?
Options: a. 3 ^ 11, b. 3 ^ 13, c. 3 ^ 25, d. 3 ^ 27, e. 3 ^ 81

Answer:
a