Starting at $8.00,as the diner lowers the price in increments of $0.20,it earns another sale.Multiply this price times this quantity to get revenue and then take the difference from the last price point to get the MR.At a price of $7.20,MR > MC but at $7.00 MR < MC,so charge $7.20.
At his price, no seniors are served and Doug can charge them a separate price if they show proper ID. The demand curve leftover for them is given by:
For seniors, at a price of $6.80, MR > MC but at $6.60 MR < MC, so charge $6.80. So Doug can offer them a $0.40 discount.