Maths
The formula behind each operation, explained with examples you can repeat by hand.
How to calculate a percentage: the formula and the four cases
A percentage is worked out three different ways depending on what you need. A percentage of an amount, what one number is of another, and percentage change.
4 min readHow to do a rule of three: direct, inverse and compound
A rule of three gives the wrong answer in four common cases. The step-by-step method for the simple, direct, inverse and compound forms, with worked examples.
14 min readSpanish working days: which rule your deadline uses
The same two dates give three different answers depending on which deadline. What a working day is in Spain, and how it differs from a calendar day in law.
10 min readThe Spanish statutory interest rate: who sets it and what it is in 2026
The Spanish statutory interest rate is 3.25 % and a budget act sets it. Who decides it, why it has not moved in four years, and the five other rates.
11 min read
The content is educational and does not constitute financial advice.