In testing the lethal concentration of a chemical found in polluted water, it is found that ...

In testing the lethal concentration of a chemical found in polluted water, it is found that a certain concentration will kill 20% of the fish that are subjected to it for 24 hours. 200 fish are placed in a tank containing this concentration of chemical, and examined one after another at the end of 24 hours. What is the probability that a.) first dead fish is the third one examined b.) the first three fish are all dead c.) none of the first four fish examined are dead