Below is the current state of the limit order book:

Order num Price Quantity Order type
1 $24 20 limit sell
2 $23 20 limit sell
3 $22 10 limit sell
4 $20 5 limit buy
5 $19 10 limit buy
6 $18 15 limit buy


a. What is the mid-price, and what is the bid ask spread? What is the
maximum number of shares one can buy or sell using a single market
order?

b. Suppose you submit a market buy order for 15 shares. What is the
average per-share price at which your order will be executed? Assuming
that no new limit orders are submitted, what will happen to the mid
price?