The total $100 + $250 = $350 never changes. In the end Fiona has $30 more than Sebastian, so removing the $30 leaves two equal parts.

Sebastian$160Fiona$160$30total $350

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

  1. Total → $100 + $250 = $350
  2. 2 equal parts → $350 − $30 = $320
  3. 1 part (Sebastian) → $320 ÷ 2 = $160
  4. Fiona → $160 + $30 = $190

Fiona has $190 left.

Check: Sebastian $160 + Fiona $190 = $350; $190 − $160 = $30. ✓