Final answer:
Holly and Ethan will need to sell 10 books to make a profit of exactly $30.
Step-by-step explanation:
To calculate the number of books Holly and Ethan need to sell to make a profit of exactly $30, we need to consider the booth rental cost and the selling price of each book.
The booth rental cost is $15, which needs to be covered before making any profit.
Therefore, we subtract this cost from the desired profit, $30, to get $30 - $15 = $15.
Next, we divide the remaining profit by the selling price of each book, $1.50, to get the number of books needed to sell.
In this case, $15 / $1.50 = 10 books.
Therefore, Holly and Ethan will need to sell 10 books to make a profit of exactly $30.