Final answer:
Gabbie started with 64 plates and had 4 left after the picnic; therefore, she used 60 plates at the picnic.
Step-by-step explanation:
Gabbie bought 8 packages of plates, and each package contained 8 plates, resulting in a total of 64 plates (8 packages × 8 plates/package = 64 plates). After the picnic, Gabbie was left with 4 plates.
To find out how many plates were used at the picnic, subtract the plates left from the total number of plates (64 plates - 4 plates = 60 plates).