(a) The online brokerage company E*TRADE has two types of customers: uninterested traders and obsessed traders. Uninterested traders have a relatively inelastic demand curve D_u. E*TRADE would like to charge uninterested traders the profit-maximizing commission rate P_u = $30 per trade and sell quantity Q_u trades per month.
(b) Obsessed traders have a relatively elastic demand curve D_o. E*TRADE would like to charge them the lower commission rate P_o = $9 per trade. Although E*TRADE cannot directly identify which group any particular trader belongs to, it can set different prices for the two groups using a quantity discount by requiring traders to make at least Q_o trades per month to get a reduced commission rate.