Everyday Maths
The mental arithmetic that saves you money in a shop, a showroom or a meeting.
One division tells you how long money takes to double. Here's why it works — and the big lesson about rates hiding inside it.
Squaring up after a group trip is not everyone paying everyone. Net each person to a single balance, match debtors to creditors, and four people need at most three payments — with the odd penny accounted for.
Some of your costs move with the headcount and some do not. Work out the marginal cost of one guest and your budget stops surprising you — with a worked example at two guest counts.
A monthly price is the wrong unit for a decision. Convert every subscription to an annual figure on the same scale and the small ones stop hiding — with a fully worked hypothetical list.
Per-head planning arithmetic for a party shopping list — mains, sides, snacks, drinks, ice and tableware. Every rate here is a starting estimate you adjust, not a rule.
A cup measures space, not mass, and the same cup of flour can hold noticeably different amounts depending on how it was filled. Why the dry conversion is ingredient-specific and can never be one number.
Two scales that do not share a zero cannot be converted by multiplying. The arithmetic behind the 1.8, the mental shortcut and exactly how wrong it gets, and the one reading where both scales agree.
The ingredients are a straightforward multiplication. The tin and the depth are geometry, and time follows the depth rather than the quantity — which is why doubling is not simply doubling.
Ounces, pints and tons all mean two or three different things depending on which side of the Atlantic wrote them down. The exact definitions, and the differences that are big enough to matter.
A rate moving from 4% to 6% rose 2 percentage points and 50 percent. Both statements are true, they are not interchangeable, and the wrong one gets quoted constantly.
Swap the two numbers and you get a different answer that still looks reasonable. Here is how to tell which number is the whole — and the difference between percent of and percent more than.
How far off was the estimate? The number you divide by decides the answer, and dividing by your own estimate quietly flatters you.