Boys = 1 unit, girls = 2 units, adults = 4 units. Adults have 2 units more than girls, and that gap is 378.

Boys1Girls11Adults1111378 more = 2 units

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

  1. Adults − Girls = 2 units → 378
  2. 1 unit → 378 ÷ 2 = 189 (boys)
  3. Children = boys + girls = 189 + 2 × 189 = 567

There are 567 children at the carnival.

Check: adults = 4 × 189 = 756; 756 − 378 (girls) = 378. ✓