Show me the steps to solve $$A market maker faces the following demand and supply for widgets. Eleven buyers are willing to buy at the following prices: $$15,$ $$14,$ $$13,$ $$12,$ $$11,$ $$10,$ $$9,$ $$8,$ $$7,$ $$6,$ $$5.$ Eleven sellers are also willing to sell at the same prices. If the market maker makes three transactions, what is his total profit?

Question $7$ options:

$$$$12$

$$$$15$

$$$$18$

$$$$21$