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? Do you know what they are called?