A researcher is testing a possible carcinogen on mice. He chooses 7 mice at random from a batch of 13 mice. Unknown to him, 5 of the mice already have cancers. What is the probability that there are 3 cancerous mice in the group he has chosen? Solution (6C2)(7C6)/(13C8) = 15*7/1287 = 105/1287 = 35/429 = 0.0816.