how to multiply two numbers close to a hundred in your head
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 number’s shortfall is how far it is below 100. The trick works for any two numbers from 51 to 99, and with excesses for any two from 101 to 199.
Near 10, 1000 or any other power of ten, it works the same way: count the shortfalls to that power of ten, write their product with as many digits as the power of ten has zeros, and add the rest of the product to the part on the left. 997 × 996: shortfalls 3 and 4, 997 − 4 = 993 thousands, 3 × 4 = 12 written as 012, so 993012. 8 × 7: shortfalls 2 and 3, 8 − 3 = 5 tens, 2 × 3 = 6, so 56.
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