how to multiply 34 × 26, or any two two-digit numbers, writing down only the answer
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.
Crosswise is the sum of two products: the tens of each number by the units of the other. A carry goes to the column on the left.
Swap the digits in both numbers: the outer columns trade places and the middle one stays the same. So if the product of the tens equals the product of the units, the product of the numbers does not change: 12 × 42 = 21 × 24 = 504, 23 × 64 = 32 × 46 = 1472. Otherwise the products differ by 99 times the difference between the product of the tens and the product of the units: 43 × 62 − 34 × 26 = 99 × 18.
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