Figure: Good X Price $400 300 200 -. . . . 100 D 50 100 150 200 Quantity of Good X As the price falls from $200 to $100, consumer surplus changes by: $12.500 $5,000 O-$25000 $10.010.base 100 area above the mark 150 150 quantity demanded height 200 price was paid 100 100 qnty was consumed 150 * 100 / 2 = 7500 base 200 - 100 = 100 height