For the good with the demand curve to the right, the 3rd unit consumed is worth 10. In other words, if consumption had been restricted to 2 units, the 3rd could be sold for a price of 10. If however 4 units were already consumed the 5th would be worth 8 and so on.
Since consumers typically pay the same price for all units consumed, they purchase as long as the price of added units is greater than their marginal willingness to pay, or the value they place on one more unit. If the price were 5 or lower at least 9 units would be purchased.
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