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: an optometrist charges $ 150 per pair for soft contact lenses and $ 85 per pair for hard contact lenses. last week she sold 5 more pairs of soft lenses than hard lenses. if her total sales for pairs of contact lenses last week were $ 1,220, what was the total number of pairs of contact lenses that she sold ?
Options: a. 9, b. 13, c. 15, d. 17, e. 19

Answer: a

Question: Problem: a cyclist bikes x distance at 8 miles per hour and returns over the same path at 10 miles per hour. what is the cyclist ' s average rate for the round trip in miles per hour ?
Options: a. 8.1, b. 8.9, c. 8.6, d. 8.3, e. 9.0

Answer: b

Question: Problem: two cyclist start from the same places in opposite directions. one is going towards north at 30 kmph and the other is going towards south 40 kmph. what time will they take to be 50 km apart ?
Options: a. 1.32 hr, b. 0.714 hrs, c. 3.231 hrs, d. 0.515 hrs, e. 6.214 hrs

Answer:
b