How many tiles does she use on a square of a side of five? A, 10, B, 11, C, 12, D, 14, or E, 16. Let's look at the tile. It has a length of two and a height of one. Now let's look at the example squares that we have. For a square of a side of one, Katrin used four tiles, one, two, three, four. It's one on each side. For a square of a side length of three, she needed to add enough tiles to make the side length two longer. So this one was one and this one's three. The difference is two. So it just takes one extra tile on each side to put the tiles all around the square. So there were four in the first one, and there are four more in the second one for a total of eight. Now with a side length of five, the length is again two more. So it'd be a tile here that's added. Think about it that way. And there are other tiles here. So now we have four extra tiles because she needs to add enough tiles to make up for this extra two units of length on each side. So there were eight tiles in the square of a side length of three, and there will be four more tiles, one on each side for the square of a side length of eight for a total of 12 tiles. So the answer is C, 12. Another way to look at this is to simply draw in the tiles. It takes two and a half tiles to cover this length of five, and we have a little bit sticking out to make the border. So we have two units here, two units here, one unit here, and then one unit here, which lines up perfectly with the other dimension of the tile. So again, we have three of them on this side. So then we have three of them on this side, three of them on this side, and three on this side for a total of 12 tiles.

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