how to find the square root of 5476 in your head, and of any perfect square
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 trick is only for perfect squares, that is, squares of whole numbers. A number ending in 2, 3, 7 or 8 is never a perfect square; otherwise, check the root by multiplying it out.
Cube roots work the same way, only simpler: the last digit of a cube fixes the last digit of the root, with no choice. 2 and 8 swap places, so do 3 and 7, and every other digit stays itself. Drop the last three digits and find the largest cube no more than what is left. 314432 leaves 314: 6³ = 216, and 7³ = 343 is already more, so the tens are 6; the last digit 2 gives 8, so the root is 68.
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