观察下面的图形。它们是由阴影瓷砖和空白瓷砖组成的。

1. Identify the pattern for the number of shaded tiles

The number of shaded tiles in Figure 1, 2, and 3 are $7$, $11$, and $15$ respectively.
The number of shaded tiles increases by $4$ for each subsequent figure.

2. Find the figure number with 19 shaded tiles

Since Figure 3 has $15$ shaded tiles, the next figure (Figure 4) will have $15 + 4 = 19$ shaded tiles.
Alternatively, using the $n$-th term formula: $4n + 3 = 19 \implies 4n = 16 \implies n = 4$.

1. Determine the side length of Figure 18

The side length of Figure $n$ is $n + 2$.
For Figure 18, the side length is $18 + 2 = 20$.

2. Calculate the total number of tiles in Figure 18

Total number of tiles $= 20 \times 20 = 400$

1. Find the number of shaded tiles in Figure 18

The number of shaded tiles forms an arithmetic sequence: $7, 11, 15, \dots$
The $n$-th term is $4n + 3$.
For Figure 18, number of shaded tiles $= 4(18) + 3 = 75$.

2. Find the total number of tiles in Figure 18

The total number of tiles in Figure $n$ is $(n+2)^2$.
For Figure 18, total number of tiles $= (18+2)^2 = 20^2 = 400$.

3. Calculate the percentage of shaded tiles

$\text{Percentage} = \frac{75}{400} \times 100\% = 18.75\%$