The answer is 640 ft².
Mr. Klampett estimates 8 ft² of storage space per individual will be required to meet the needs of 12 people. So, for 12 people, the storage space is:
8 ft² * 12 = 96 ft²
If the storage space of 96 ft² is 15% of the total amount of space required, x will be 100%:
96 ft² : 15% = x : 100%
x = 96 ft² * 100% : 15%
x = 640 ft²