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Mid after a sweep

Market makingMedium

The best bid is $100$ . The ask side holds $10$ units at $101$ , $20$ units at $102$ and $30$ units at $103$ . A market order buys $25$ units, filling against the best prices first, and nothing else changes. What is the mid price of the best bid and the best ask afterwards?

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Hint

Find which ask level is left after the order.

Solution

The order clears the level at $101$ and takes $15$ units from the level at $102$ .
The new best ask is $102$ , with $5$ units left.
The new mid price is $(100 + 102) \div 2 = 101$ .

Answer: $101$

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