Where <em>x </em>is the number of books sold, <em>i </em>is income, and <em>c </em>is cost:
<em>c = 4x + 3500 </em> ($4 per book plus the 3500 flat marketing fee)
<em>i = 15x </em>($15 per book)
You are looking for the point where these intersect: the intersection is where the income is equal to the cost, and at any point after that the income is greater than the cost. So, set the equations equal to each other:
<em>4x + 3500 = 15x </em>
subtract 4x from both sides
<em>3500 = 11x</em>
divide both sides by 11
<em>x = 318.181818</em>
So, you would have to sell a minimum of 319 books in order to make a profit.
