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.