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## 'What's Left?' printed from http://nrich.maths.org/

Here is a grid showing the numbers $1-20$:

Start with the first red number, $2$. Now, look along the top
row of the grid to the next number, $3$.

Can you share $3$ exactly between $2$?

If the answer is yes then shade in the $3$ box. If the answer is
no, leave it.

Now take the next number, $4$. Can you share $4$ exactly between
$2$?

Again, if you can, shade the box with $4$ in it. Carry on doing
the same with all the numbers.

Now do the same with the next red number, $3$. Look along the
row for the next number, $4$ and shade all the numbers that will
share exactly between $3$.

Now try with $4$ and with $5$.

Which numbers do you have left?