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## 'Reach 100' printed from http://nrich.maths.org/

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.