The barbs are 7 equal units. Three units are female, so the other 4 units are male = 12.

Tiger barbs3333333male 12 = 4 units

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

  1. Male = 4 units → 12
  2. 1 unit → 12 ÷ 4 = 3
  3. Total = 7 units → 7 × 3 = 21

In total Max bought 21 tiger barbs.

Check: female = 3 × 3 = 9; 9 + 12 = 21. ✓