In a class, $40 \%$ of the students have poor eyesight. Of these, $70 \%$ wear glasses and the remaining $30 \%$ wear contact lenses. Knowing that 21 students wear glasses, how many students are there in this class?
Let $A$ be the total number of students in the class. Thus, $\frac{40}{100} \times A$ do not see well. Therefore, $\frac{70}{100} \times \frac{40}{100} \times A$ wear glasses. Consequently, we have:

$$
\frac{70}{100} \times \frac{40}{100} \times A=21 \Rightarrow A=\frac{21 \times 100}{7 \times 4}=3 \times 25=75
$$