Look at the figures below. They are made up of shaded tiles and plain tiles.

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

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