(7) From $\{1,2,3, \cdots, 100\}$, if 5 numbers (repetition allowed) are randomly selected, then the expected number of composite numbers is $\qquad$ .
(7) $\frac{37}{10}$ Hint: Among $\{1,2,3, \cdots, 100\}$, there are 74 composite numbers. Let $\xi$ be the number of composite numbers drawn, then
$$
P(\xi=i)=\mathrm{C}_{5}^{i}\left(\frac{74}{100}\right)^{i}\left(\frac{26}{100}\right)^{5-i}(0 \leqslant i \leqslant 5),
$$

Thus, $\xi$ follows a binomial distribution.
Therefore, $E \xi=5 \times \frac{74}{100}=\frac{37}{10}$.