Follow the clues to find the mystery number.
What do the digits in the number fifteen add up to? How many other
numbers have digits with the same total but no zeros?
A church hymn book contains 700 hymns. The numbers of the hymns are
displayed by combining special small single-digit boards. What is
the minimum number of small boards that is needed?
David sent in this quite neat explanation:
For example, in a 3x3 square, the number in
the top right is 2 more than the number in the top left. The number
in the bottom left is 2 less than the number in the bottom right.
So when we add the top left and bottom right, and the top right and
bottom left, we get the same total.
Esther who is 8 used letters to stand for the
numbers (we call this algebra) and shows why the totals are the
Thank you Esther, that's a very clear way of
Even if you haven't met algebra, you can still
use ordinary words as David did. Well done!