how to multiply two numbers when all you can do is halve, double and add
Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.
Three sizes in one archive — from a sheet on the fridge to a poster on the wall.
When halving, drop the remainder: 37 → 18 → 9. A crossed-out row has an even left number, and its right number stays out of the sum.
More than three and a half thousand years ago the scribe Ahmes copied out a mathematical scroll, noting that he was working from an ancient copy made in the time of the pharaoh Amenemhat III. It shows multiplication done just like the Egyptian way above: by doubling and adding. In 1858 the Scottish antiquarian Henry Rhind bought two parts of the scroll in Luxor, so it is called the Rhind papyrus, though more and more it is named after the scribe, Ahmes.
Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 2 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.
the IWF colour code and the plate count: from target weight to each side of the bar, no calculator
why a “yes” from a test that is right 99 times in 100 is wrong more than nine times in ten when one in a thousand has the thing
the first digit of a number: why a one turns up more than six times as often as a nine, and in which sets that is true
how many people it takes for two of them to share a birthday — and the same count for codes and fingerprints