Given a simple high-school level math question, you are required to solve it and provide the final answer. The final answer is always a single number. These questions can range from a variety of topics like simple arithmetic, solving equations, converting a quantity from one unit to another, finding remainders/GCD/LCM, finding probabilities etc. Each question has only one correct answer. This answer can be a positive or negative integer, a fraction or a decimal number. If the answer is a negative number use the hyphen (e.g. -42) symbol for the minus sign. For decimal numbers, do not add extra zeros after the decimal point. For fractional numbers, separate the numerator and denominator using a forward slash (e.g. 3/25).
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Question: What is prob of sequence rrr when three letters picked without replacement from bbbrbbbbbwrrwrrrrr?

Answer: 7/102


Question: Four letters picked without replacement from {q: 1, h: 1, u: 1, e: 1, r: 1}. Give prob of picking 2 q, 1 h, and 1 r.

Answer: 0


Question: Suppose -4*v = -a - 409, 480 = -5*v + 10*v + 5*a. What is the remainder when v is divided by 21?

Answer:
17