Mental Arithmetic Guides
Everything here is about doing arithmetic in your head faster than you can reach for a phone — not as a party trick, but because a number you can estimate is a number you can check, and almost every expensive mistake with figures is one nobody challenged.
The guides stand alone and can be read in any order. If you are starting from scratch, work down the list.
Start here
Mental Math Techniques
The core set: work left to right, round and correct, subtract by counting up, halve one side and double the other. If you read only one of these, read this one.
Adding and Subtracting in Your Head
Why the paper method is the hard way round, how to stop borrowing across columns entirely, and where wrong answers actually come from under time pressure.
The Times Tables That Actually Matter
The twelve-by-twelve grid is 144 facts. Strip out the rules and the repeats and roughly fifteen are genuinely worth memorising. Here they are, and here is what to use instead of the rest.
Going further
Multiplying in Your Head
Splitting, rounding and correcting, difference of squares, and the squares worth knowing outright. Turns one hard multiplication into two or three easy ones.
Percentages Without a Calculator
Everything from ten per cent, the swap that saves the most time, stacked discounts, and the reversal mistake nearly everybody makes.
Estimating: The Skill Behind the Skill
The rough answer is the important one. How to round so errors cancel, how to keep digits and magnitude apart, and why estimating is what catches a slipped decimal point.
Putting it to use
Mental Maths for Everyday Money
Bills, tips, discounts, unit prices and the rule of 72. The half-dozen shapes that cover almost all the arithmetic an adult actually does.
Why Mental Arithmetic Still Matters
The case for bothering at all when everyone has a calculator, what a realistic standard looks like, and why a clock helps more than repetition alone.
And then practise
Reading these is the easy part. The gap between understanding a method and reaching for it under pressure only closes with repetition, and short sessions beat long ones.
Carry the One is built for that: three difficulties, four answers where one is right, and a clock that shortens as you climb. How to play covers the rules.