Here is a square sequence of numbers.

392?
6  3
7  4
?817

What are the two question marks?

The complete sqare is as follows :
3921
6 3
7 4
9817
Reading clockwise, each number is simply the sum of the previous two, modulus 10. E.g. 1 + 3 gices 4, 7 + 6 gives 3 (from 13, ignoring the initial 1).
It is interesting to note that opposite digits always sum to 10. The missing digits comply with this rule - though it is of no use in deducing them.

Posted by Richard Briscoe on 2003-09-09 11:28:58