At Sugar Creek Middle School, there are two sizes of lockers for the students: one size for the sixth-grade and seventh-grade students and a larger size for the eighth-grade students. Both sizes of lockers are 5 feet tall and 1 foot wide. The lockers for the younger students each have a volume of 5 cubic feet, while the lockers for the eighth-grade students each have a volume of 7.5 cubic feet.
How much deeper are the lockers for the eighth-grade students than the lockers for the younger students?