So we want to know what percentage of bees would survive a virus after 19 days of decay if the bee population has a half life of 5 days. So every 5 days 50% of bees die. Lets say that N is the number of bees before the virus. So after 5 days is 50% less bees, so 50% of N remains. After another 5 days again, 50% bees die, and 50% out of 50% is 25%. So after 10 days, 25% of bees remain or 25%N or 0.25*N. After another 5 days its 12.5 % of bees remain or 0.125*N. And after 4 days 40% more bees die. And that is 0.4*0.125*N = 0.05N. 0.05*100% is 5%. So after 19 days, 5% of bees remain and 95% of bees is dead.
