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Consumer surplus is an economic measurement of consumer benefits. A consumer surplus happens when the price consumers pay for a product or service is less than the price they're willing to pay. It's a measure of the additional benefit that consumers receive because they're paying less for something than what they were willing to pay

From the same website, Investopedia has an example:

For example, suppose consumers are willing to pay 50 dollars for the first unit of product A and 20 dollars for the 50th unit. If 50 of the units are sold at $20 each, then 49 of the units were sold at a consumer surplus, assuming the demand curve is constant.

It is really strange for me, it should be that only the first unit is sold at a consumer surplus because the consumer is willing to pay for $20 for the last 49 units, similar to the sold price, only the price of the first unit is different.

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    $\begingroup$ (-1) The quoted text does not say that "the consumer is willing to pay for $20 for the last 49 units, similar to the sold price, only the price of the first unit is different". Or did you mean to write "could" instead of "should"? $\endgroup$
    – Giskard
    Aug 17 '21 at 8:34
  • $\begingroup$ @Giskard . I think the quoted text said the same as my summary. "If 50 of the units are sold at $20 each, then 49 of the units were sold at a consumer surplus" means that all 50 units will be sold at 20 while the consumer is willing to buy 50 for the first item and 20 for the last 49 items $\endgroup$
    – Louise
    Aug 17 '21 at 8:39
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    $\begingroup$ "all 50 units will be sold at 20" is correct, "the consumer is willing to buy 50 for the first item and 20 for the last 49 items" is incorrect, and I don't know where you get this interpretation. $\endgroup$
    – Giskard
    Aug 17 '21 at 8:40
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    $\begingroup$ ahhh, I see, do you mean that the first 49 units will be sold at the consumer surplus? $\endgroup$
    – Louise
    Aug 17 '21 at 8:41
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    $\begingroup$ P.s.: I generally dislike Investopedia, because it is not precise and confounds things. It is not a good source in my opinion. (It also accepts advertisments from rather dubious businesses.) $\endgroup$
    – Giskard
    Aug 17 '21 at 8:46
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I know this question has almost definitely been answered in the comment section, yet I will try my best to clarify the theoretical background.

"Willingness to Pay", or WTP for short, is each individual consumer's maximum price at which they are willing to buy one unit of a commodity. By an axiomatic admission, we take it that the more of an item you want to buy, the less you wish to pay per item. This is shown in the negative slope of an elemental (and constant) demand curve.

Say you want to buy a book, with the prices shown in the picture below. The first unit can be sold to Aleisha for \$59, but won't, if the market is perfectly competitive and sellers cannot make price discriminations. This means that every unit will be sold to everybody for the same amount of cash. Next on the list you put Brad, who wants to buy the book but has a WTP of \$45, less than that of Aleisha. We make the same argument about Claudia, and then we stop at Darren: He is willing to pay less than the asking price, and so he won't buy a unit. Thus, the final quantity sold will be 3 units, determined by the balance between WTPs and asking price.

enter image description here

If we were to make this procedure continuous instead of discrete, we will inevitably reach a point of equilibrium between WTP and price. At that point, the final person to purchase the product would have done so exactly at his WTP - all others would have either a large or a marginal consumer surplus.

Back to the aforementioned example, we have to agree that it is correct: the final unit (1) was bought for no surplus, while all others (49) were bought at a substantial surplus.

Sources: Hal Varian - Microeconomic Analysis

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