Again consider the same graph. We know that her total expediture for the tapes was $5 x 4 = $20. This is also the area of the red rectangle that represents expenditure.

    We can also compute the dollar value of her consumer surplus from this same graph. (Remember that the area of a triangle is 1/2 x base x height.) The value of her consumer surplus is the area of the green triangle, $4 high ($9 - 5) and a base of 4 (videotapes):
CS= 1/2 x 4 x 4 = $8.

    Notice that we express consumer surplus as a monetary amount. This is important to remember, consumer surplus is a very real monetary measure of value.

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