Girls = 9 units, boys = 8 units. There are 99 more girls, which is the 1-unit difference.

BoysGirls1 unit = 99

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

  1. Difference → 9 units − 8 units = 1 unit → 99
  2. Total → 9 + 8 = 17 units = 17 × 99 = 1683

There are 1683 pupils in the school.

Check: girls 891, boys 792; 891 − 792 = 99; 891 + 792 = 1683. ✓