Split the 40 marbles into 10 equal parts (each = 10% = 4 marbles). He gave away 60% = 6 parts, so 40% = 4 parts are left.

40 marbles4444444444gave away 60%left 40%

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

  1. 100% → 40, so 10% → 40 ÷ 10 = 4 marbles
  2. Left → 100% − 60% = 40% = 4 parts
  3. 40% → 4 × 4 = 16

Johnston had 16 marbles left.

Check: gave 60% × 40 = 24; 40 − 24 = 16. ✓