The value of the invoice on May 18 is $3200.
Amount of invoice on May 17 = $4000
Percentage decrease = 20%
Decrease in amount = 20% × $4000 = 0.2 × $4000 = $800
Therefore, the value of the invoice on May 18 will be:
= Amount of invoice on May 17 - Reduction in the amount
= $4000 - $800
= $3200
Therefore, the value of the invoice on May 18 is $3200
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