Split the 15 pies into 3 equal units. Two units were sold, so 1 unit is left.

15 pies555left = 1 unit

Plain numbers are given; italic underlined numbers are worked out.

  1. 3 units → 15
  2. 1 unit → 15 ÷ 3 = 5
  3. Left = 1 unit → 5

5 pies were left.

Check: 15 − 10 sold = 5. ✓