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: Calculate prob of sequence dqd when three letters picked without replacement from fqfuurdttdft.

Answer: 1/660


Question: Solve -i + z + 1 = 0, 17 = 2*i - 7*z + 2*z for i.

Answer: -4


Question: Suppose 0*x - x - 9 = 0. Let b = -2 - x. What is the units digit of b?

Answer:
7