Brother = 1 unit, Sean = 4 units, so 5 units = $100 (given). Each $20, the $50 share and the $30 to give are worked out.

Brother$20Sean$20$20$20$20equal share $50give $30$100

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

  1. 5 units → $100
  2. 1 unit → $100 ÷ 5 = $20 (brother), Sean = $80
  3. Equal share → $100 ÷ 2 = $50 each
  4. Sean gives → $80 − $50 = $30

Sean must give his brother $30.

Check: Sean $80 − $30 = $50; brother $20 + $30 = $50. ✓