Answer: After 5 hours, Mary will be at the break even point.
To answer this question, we need to find the time when the cost equals the revenue.
Therefore, set our equations equal to each other and solve.
20 + 30h = 10h^2 - 80
0 = 10h^2 - 30h - 100
0 = h^2 - 3h - 10
0 = (h - 5)(h + 2)
Our solutions are h = 5 or h = -2. Only 5 hours makes sense in the context of this problem.