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: there are 15 balls in a jar. you take out 3 blue balls without putting them back inside, and now the probability of pulling out a blue ball is 1 / 3. how many blue balls were there in the beginning ?
Options: a. 12., b. 9., c. 8., d. 7., e. 6.

A: d
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Q: Problem: if the ratio of two number is 6: 7 and lcm of the number is 450 then what is the number.
Options: a. 11, b. 20, c. 25, d. 30, e. 35

A: a
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Q: Problem: a 600 meter long train crosses a signal post in 30 seconds. how long will it take to cross a 3 kilometer long bridge, at the same speed ?
Options: a. 4 min, b. 2 min, c. 8 min, d. 3 min, e. 5 min

A:
d
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