how to multiply any positive whole number by 11 in your head, digit by digit
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.
A digit’s neighbour is the digit to its right. Works for any positive whole number, of any length.
The rule of adding each digit to its neighbour on the right is part of the mental arithmetic system of Jakow Trachtenberg, a mathematician and engineer born in Odesa in 1888. He developed it to keep his mind occupied while held prisoner in a Nazi concentration camp, and later fled to Switzerland. A book on the system came out in English in 1960.
Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 5 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