The producer surplus is 64.
Buyers of his service are willing to pay $40 per hour for as many hours Allen is willing to tutor.
On a particular day, he is willing to tutor the first hour for $10, the second hour for $18, the third hour for $28, and the fourth hour for $40.
= (40 - 10) +(40-18) + (40 - 28) + (40 - 40)
= 30 + 22 + 12 + 10
= 64
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