Math
Fast percentage, increase and discount math, explained step by step.
Percentages are the maths people use most outside school and get wrong most often. Removing an included percentage is not subtracting it; two stacked discounts do not add up; and moving into a higher tax band never leaves you earning less.
This is also where the cuerda converter lives, solving a problem that exists only in Puerto Rico: the cuerda is the island's land measure, it looks a lot like an acre without being one, and that difference turns into real money as soon as the parcel gets big.
Almost all of these sums get done from memory and in a hurry, which is exactly when they fail. Each calculator here shows the intermediate step, not just the answer: seeing why you divide instead of subtracting is what means you won't need the tool next time.