Cut the 70 people into 10 equal units (each = 7). Adults take 3/10 = 3 units. The remainder is 7 units; 1/7 of it (1 unit) are boys, the other 6 units are girls.

70 people7777777777adultsremainder (1 boy + 6 girls)

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

  1. 1 unit → 70 ÷ 10 = 7 people
  2. Adults → 3 units = 21; remainder = 7 units = 49
  3. Boys → 1/7 of 49 = 1 unit = 7
  4. Girls → 6 units = 6 × 7 = 42

There are 42 girls.

Check: 21 adults + 7 boys + 42 girls = 70. ✓