24 classes for Max (August, September, and prior invoice correction +9 classes)

August 9 classes

September 9 classes

minus 3 classes missed by the teacher from the last invoice

18-3 = 15 classes

Max – should have been invoiced for 9 classes in June and 9 classes in July. The invoice sent on June 22nd was issued for only 5 classes in June and 4 in July. 9 classes were mistakenly not invoiced.

17 classes plus 9 classes mistakenly omitted from the previous invoice = 26 classes total.