THE COLLATZ CONJECTURE
halve the even, triple the odd and add one: the road of 27, the record holders and a problem nobody has solved
Free
DIVISIBILITY TESTS
what to read in a written number instead of dividing it: the tail, the digit sum and composite divisors
Free
THE DOOMSDAY RULE
the weekday of any date in the Gregorian calendar, worked out in the head in three steps
Free
NUMBERS 1 TO 100
the hundred chart · primes picked out in colour
Free
LEAP YEARS
one rule, a cycle of four hundred years and the ninety-seven extra days in it
Free
MULTIPLICATION TABLE
1 to 12 · colour intensity = size of the product
Free
THE RULE OF 72
the time it takes steady growth to double: the division you can do in your head, how far it misses and why the divisor is what it is
Free