Cut the 35 pies into 7 equal units (sevenths). 4 units were sold; 3 units are left.<br><br>[[model:questions/solutions/q3400.png]]<br><br>• 7 units → 35 pies<br>• 1 unit → 35 ÷ 7 = 5<br>• Left → 3 units = 3 × 5 = 15<br><br>15 pies were left.<br>Check: sold 20 + left 15 = 35. ✓