how to multiply 43 × 47 in your head, and every pair like it
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.
The tens are the whole number without its last digit, and the units are the last digit. The trick works for any two numbers with the same tens whose units add up to 10.
Two-digit numbers have a mirror trick too: the units are the same and the tens add up to 10. Then the hundreds are the product of the tens plus the shared units digit, and the product of the units goes after them as two digits. For 26 × 86 the hundreds are 2 × 8 + 6 = 22, and 6 × 6 = 36 goes after them, so 2236. For 41 × 61 the hundreds are 4 × 6 + 1 = 25, and 1 × 1 = 1 goes after them as 01, so 2501.
Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 7 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