By doing dimensional analysis, we can easily calculate for what is being asked here. We do as follows:
Given:
Energy per time = 2400 BTU/hr
time period = 3 hrs
number of subjects = 7
Measurement of the weights = 20 ft × 25 ft × 12 ft
The energy produced for 3 hours would be:
2400 BTU/hr ( 3hr) = 7200 BTU
Volume of each weight = 20 ft × 25 ft × 12 ft = 6000 ft^3
Energy per volume = 7200 / 6000 = 1.2 BTU/ft^3
However, we have 7 bodybuilders producing this amount therefore,
Total energy produced = 7 (1.2 BTU/ft^3) = 8.4 BTU/ft^3