The bacteria in a lab dish double in number every four hours. If 500 bacteria cells are in the dish now, in how many hours will there be exactly 32,000 bacteria?
32000 bacteria is $32000/500=64$ times the number currently in the lab dish. Since $64=2^6$, the bacteria had to double 6 times to reach this number. Since the bacteria double every four hours, it takes $4\cdot6=\boxed{24}$ hours.