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).

[Q]: Solve -k = -4*a + 15, 0 = 6*k - 10*k + a + 15 for k.
[A]: 5


[Q]: What is 0.7 take away 271816?
[A]: -271815.3


[Q]: Calculate prob of sequence xa when two letters picked without replacement from {a: 2, x: 5}.
[A]:
5/21