Choose any three by three square of dates on a calendar page...
Make a set of numbers that use all the digits from 1 to 9, once and
once only. Add them up. The result is divisible by 9. Add each of
the digits in the new number. What is their sum? Now try some other
possibilities for yourself!
What happens to the perimeter of triangle ABC as the two smaller
circles change size and roll around inside the bigger circle?
These are top-heavy pyramids. The blue one has $21$ at the apex
(top) and the red one has $31$.
A pair of numbers are added to make the number
above that pair.
In the blue top-heavy pyramid whose base is $4$,
$5$ and $7$, $4 + 5 = 9$, so $9$ is placed between and above the
$4$ and the $5$.
$5 + 7 = 12$ and $9 + 12 = 21$.
Use the numbers in the box below to make the base
of a top-heavy pyramid whose top number is $200$.