Some nomadic tribes, when faced with a life threatening, contagious disease, try to improve their chances of

Question:

Some nomadic tribes, when faced with a life threatening, contagious disease, try to improve their chances of survival by dispersing into smaller groups. Suppose a tribe of twenty-one people, of whom four are carriers of the disease, split into three groups of seven each. What is the probability that at least one group is free of the disease?