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).

In base 9, what is -356 + -2?
-358

Calculate prob of sequence tw when two letters picked without replacement from {t: 5, p: 1, y: 6, w: 4, h: 1, s: 3}.
1/19

Suppose 5*a - 4*k - 63 = 0, -7*a + 3*k = -11*a + 69. Calculate the greatest common divisor of 15 and a.

15