19. (12 points) In response to the national strategy of "targeted poverty alleviation, industrial poverty alleviation", Harbin City is recruiting volunteers for the voluntary promotion of the "Poverty Alleviation Policy" across the city. From 500 volunteers aged between $[20,45]$, 100 are randomly selected, and their age frequency distribution histogram is shown in Figure 5.
(1) Find the value of $x$ in the figure;
(2) Among the 100 selected volunteers, 10 are chosen using stratified sampling based on age to participate in the promotional activities at the central square. Then, 3 of these 10 volunteers are selected to serve as main leaders. Let $X$ be the number of these three volunteers who are under 35 years old. Find the distribution of $X$ and its expected value.
19. (1) According to the frequency distribution histogram, we have
$$
(0.01+0.02+0.04+x+0.07) \times 5=1 \text {. }
$$

Solving for $x$ gives $x=0.06$.
(2) Using stratified sampling, 10 volunteers are selected from 100, among which, 6 are under 35 years old, and 4 are 35 years old or older. Therefore, the possible values of $X$ are $0, 1, 2, 3$.
$$
\begin{array}{l}
\text { Then } P(X=0)=\frac{\mathrm{C}_{4}^{3}}{\mathrm{C}_{10}^{3}}=\frac{1}{30}, \\
P(X=1)=\frac{\mathrm{C}_{6}^{1} \mathrm{C}_{4}^{2}}{\mathrm{C}_{10}^{3}}=\frac{3}{10}, \\
P(X=2)=\frac{\mathrm{C}_{6}^{2} \mathrm{C}_{4}^{1}}{\mathrm{C}_{10}^{3}}=\frac{1}{2}, \\
P(X=3)=\frac{\mathrm{C}_{6}^{3}}{\mathrm{C}_{10}^{3}}=\frac{1}{6} .
\end{array}
$$

Thus, the distribution of $X$ is as shown in Table 2.
Hence, $\mathrm{E}(Y)=0 \times \frac{1}{30}+1 \times \frac{3}{10}+2 \times \frac{1}{2}+3 \times \frac{1}{6}=1.8$.