Sam and David invest $13,500 in equipment to print books. Each book costs $7.50 to print and sells for $21.00. How many books must they sell before their business breaks even?
The initial investment cost of the equipment is $13,500.
The cost to print each book is $7.50.
The selling price of each book is $21.00.
Let P be the number of books they must sell before their business breaks even.
Their total cost to print P books is 7.5P.
Their total revenue from selling P books is 21P.
Since their business breaks even when their total cost is equal to their total revenue, their equation is 7.5P=21P-13500.
Combining like terms, we get 13.5P=13500.
Dividing both sides by 13.5, we get P=<<1000=1000>>1000. Answer: \boxed{1000}.