how to multiply multi-digit numbers with single-digit products and addition
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.
Do not skip a zero in a cell: 2 × 3 is 06. A small “+1” or “+2” is a carry, already included in the digit beside it.
Italians called the method gelosia and Arabs called it shabakh, and both words mean the metal grille over a window. It is sometimes wrongly said to have been described by Fibonacci, but the similar grid in his Liber Abaci has no diagonals: each cell holds only the lower digit, and the higher one is kept in mind or written elsewhere. The diagonal gives every higher digit its own place, so the cells can be filled in any order. Napier’s bones (1617) are built on the same principle.
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