An experiment is designed to test the potency of a drug on 20 rats. Previous animal studies have shown that a $10-\mathrm{mg}$ dose of the drug is lethal $5 \%$ of the time within the first 4 hours; of the animals alive at 4 hours, $10 \%$ will die in the next 4 hours.
What is the probability that 0 rats will die in the 8-hour period?