Total students = 88. Primary : Secondary = 7 : 4.<br><br>
[[model:questions/solutions/q10501.png]]<br><br>
Total units = 7 + 4 = 11 units<br>
&bull; 11 units → 88<br>
&bull; 1 unit → 88 ÷ 11 = 8<br>
&bull; Primary = 7 units = 7 × 8 = 56<br>
&bull; Secondary = 4 units = 4 × 8 = 32<br><br>
Now find those who bring lunch in each group:<br>
Primary &mdash; bring : buy = 5 : 2, so bring (primary) = \( \frac{5}{7} \) × 56 = 40<br>
Secondary &mdash; bring : buy = 1 : 3, so bring (secondary) = \( \frac{1}{4} \) × 32 = 8<br><br>
Total students who bring lunch = 40 + 8 = 48<br>
Check: Buy primary = 16, buy secondary = 24. 40 + 8 + 16 + 24 = 88. ✓<br><br>
Final Answer: 48