Answer:
They must sell 40 shirts to break even
Explanation:
The break-even point is that point where the cost and revenue graphs intersect.
Up front,
expenditures are $280 + ($3.99)t, and this is to be set equal to revenue: ($10.99)t:

Begin solving this by subtracting ($3.99)t from both sides of this equation. The result is


($7/shirt)t = $280.
Dividing both sides by ($7/shirt), we get

t = $40
Revenue will equal costs (expenses) when these students have sold 40 shirts.
Therefore,they must sell 40 shirts to break even