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).

Suppose 4*n + 20 = -0*n. Let j = -5 - n. Let m be (60/4)/(1 - j). Calculate the highest common divisor of 15 and m.
15

What is prob of picking 3 t and 1 b when four letters picked without replacement from {t: 5, b: 3}?
3/7

What is twenty-one fifths of a litre in millilitres?

4200