3. During the New Year festival, the school bought a chocolate bar for each student. It is known that if the chocolate bars are packed 20 to a box, then 5 more boxes are used compared to when they are packed 24 to a box. How many students are there?
3. If each tray contains 20 pieces and requires $x$ boxes per week, then, $20 x=24(x-5)$. Solving for $x$ gives $x=30$. Therefore, the number of students is $20 \times 30=600$.