1. From the set $\{1,2, \cdots, 100\}$, draw five numbers with replacement. Then the expected number of composite numbers drawn is $\qquad$ .
1. 3.7

Analysis: It is easy to know that there are 74 composite numbers in $\{1,2, \cdots, 100\}$, so we have $E X=\sum_{i=1}^{5} i \cdot C_{5}^{i} \cdot\left(\frac{37}{50}\right)^{i} \cdot\left(\frac{13}{50}\right)^{5-i}$ $=\sum_{i=1}^{5} 5 \cdot C_{4}^{i-1} \cdot\left(\frac{37}{50}\right)^{i} \cdot\left(\frac{13}{50}\right)^{5-i}=\frac{37}{10} \cdot \sum_{i=0}^{4} C_{4}^{i} \cdot\left(\frac{37}{50}\right)^{i} \cdot\left(\frac{13}{50}\right)^{4-i}=3.7$