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?
Simply writing out the numbers $1 - 9$ along the top of your
paper can be a great help.
Imagine arranging your four two-digit numbers in columns to do
the addition sum, like this:
$5 \: 2$
$1 \; 9$
$5 \; 1$
$2 \; 9$
If the total has to be $100$, think about how the four digits
you choose could add to make the $0$ in the units column, then look
at the tens column.