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: for the past n days, the average ( arithmetic mean ) daily production at a company was 50 units. if today ' s production of 110 units raises the average to 55 units per day, what is the value of n ?
Options: a. 9, b. 11, c. 13, d. 15, e. 17

A: b
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Q: Problem: there are 750 male and female participants in a meeting. half the female participants and one - quarterof the male participants are democrats. one - third of all the participants are democrats. how many of the democrats are female ?
Options: a. 75, b. 100, c. 125, d. 175, e. 225

A: c
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Q: Problem: the number of people who purchased book a is twice the number of people who purchased book b. the number of people who purchased both books a and b is 500, which is twice the number of people who purchased only book b. what is the number of people c who purchased only book a ?
Options: a. 250, b. 500, c. 750, d. 1000, e. 1500

A:
d
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