The figure below shows cost (blue) and revenue (black) functions over the domain $0 (in thousands of units).

(Click on the graph to get a larger version.)

For what production levels is the profit function positive?
(Give your answer as an interval or a list of intervals, e.g., (-infinity,8] or (1,5),(7,10) .)

For what production levels is the profit function negative?
(Give your answer as an interval or a list of intervals, e.g., (-infinity,8] or (1,5),(7,10) .)

Estimate the production at which profit is maximized. $q \approx$ thousands of units.