3) In a school, $40 \%$ of the students have vision defects, of which $70 \%$ use only glasses and $30 \%$ only contact lenses. Knowing that there are 42 pairs of glasses in total, which of the following statements is true?

(A) 90 students have vision defects

(B) 60 students see well

(C) the school has 200 students

(D) 20 students have contact lenses

(E) none of the above
3) The answer is (E). If 42 glasses correspond to $70\%$ of all students with vision defects, it means that these are $\frac{42}{0.7}=60$. They represent $40\%$ of the students, who are thus $\frac{60}{0.4}=150$, 90 of whom see well. The students with contact lenses are $30\%$ of 60, that is 18.