Answer
5, Number of Plain Tiles = n/2+2
Work Step by Step
To find the number of plain tiles reference the image on page 3. Count the number of plain tiles, which should be 5. From the image, you will notice that there are always two spare tiles on either side of the plain tiles bordered by the name inscribed tiles. You will also notice, that excluding the two plain tiles that border the others, for every plain tile there are 2 name inscribed tiles. Therefore, for every name inscribed tile, there are 0.5 plain tiles. Finally substitute n for the number of name inscribed tiles, and you will get the final equation:
Number of Plain Tiles = n/2+2