A firm is developing a new technology to produce roofing shingles more cheaply. The annual demand for such shingles is Q=100-p. The existing industry produces at marginal cost of 50. The new technology would reduce marginal cost to 40. There are no fixed costs in shingle production using either old or new technology. A patent lasts for T years, and (to simplify calculations) the interest rate is 0 (so that we don't have to discount future years). Unfortunately, the world will explode in 20 years (so you don't have to calculate infinite series, either)

Now suppose that the existing technology is monopolized and the inventor is another firm. Calculate the profits for the potential entrant of getting this innovation, and the increase in welfare from the innovation. Assume that there will be Cournot competition following the invention.