Market Demand: Horizontal Summation of Individual Demand Curves Abe and Betty’s demand schedules (the table) and their individual demand curves (the graph) for computer games are shown. Abe will purchase 15 computer games when the price is $80, buy 25 games when the price falls to $40, and buy more as prices continue to fall. Betty will purchase 5 computer games when the price is $80 and buy 15 when the price falls to $40. The individual demand curves for Abe and Betty are shown as Da and Db, respectively, and are horizontally summed to get market demand, DMkt. Horizontal summation involves adding together the quantities demanded by each individual at each possible price.