According to a study, 50 % of adult smokers started smoking before 21 years old. 7 smokers 21 years old or older are randomly selected, and the number of smokers who started smoking before 21 is recorded. 1. The probability that at least 3 of them started smoking before 21 years of age is

2. The probability that at most 5 of them started smoking before 21 years of age is

3. The probability that exactly 2 of them started smoking before 21 years of age is