Picked Off on a Stale Quote

Fair value rises from $80$ to $81$ while your quote stays at $79.80/80.20$ . A customer buys $100$ units at your stale ask. Marking to the new fair value, what is your P&L?

Quant interview-style problem · QS-MM-008

Your answer

Think first. Use hints deliberately. Study the reasoning only when you need it.