The whole is 100%. He gave 40% to his brother, leaving 60%. He sold half of that 60% (= 30%), so the 18 cards left are the last 30%.

brother 40%sold 30%left 30% = 18100% = ?

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

  1. Remaining after brother → 100% − 40% = 60%
  2. Sold → 1⁄2 of 60% = 30%
  3. Left → 60% − 30% = 30% → 18 cards
  4. 10% → 18 ÷ 3 = 6
  5. 100% → 10 × 6 = 60

He had 60 cards at first.

Check: brother 24 + sold 18 + left 18 = 60. ✓