Failed = 25% = 1 unit; passed = 75% = 3 units. Passed has 2 units more than failed, and that gap is 16.

Failed1Passed11116 more = 2 units

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

  1. Passed − failed → 3 units − 1 unit = 2 units → 16
  2. 1 unit (failed) → 16 ÷ 2 = 8

8 pupils failed the test.

Check: passed 24 − failed 8 = 16. ✓