How to determine the original investment amount solely from the interest amount — using algebra.

How a trigonometric integral is evaluated incorrectly — and how to fix it.

How to evaluate the least positive residue using brute force, Fermat’s Little Theorem and Chinese Remainder Theorem.

How to solve a system of linear equations, when a coefficient of a variable is itself a variable.

How to calculate the probability of randomly filling N balls into k boxes, by looking at the case of 2 boxes, 3 boxes, and the general case of k boxes.