Explanation:
we need to find the number n of sold boxes, so that the income (sales) and costs (expenditures) are equal. this is called the break even point. any more boxes sold will then result in a profit, actually gaining money. and if she cannot reach that number of sold boxes, then she will make a loss.
so,
n×11 = n×3 + 24
n×8 = 24
n = 3
it will take 3 sold boxes of cards to break even.
at that point she will have spent $24 + 3×$3 = $33, and her sales will be 3×$11 = $33.