why many different three-digit numbers lead to the same 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.
A three-digit number is one from 100 to 999. The trick works with every one whose first and last digits are different: 810 of them.
The trick’s own answer, multiplied by 9, gives itself written backwards: 1089 × 9 = 9801. Among all four-digit numbers, only one other turns into its own reverse when multiplied by a number from 2 to 9: 2178, which is 1089 × 2, and 2178 × 4 = 8712.
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