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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?