State three conditions which must be satisfied for a situation to be modelled by a binomial distribution.
George wants to invest some of his monthly salary. He invests a certain amount of this every month for 18 months. For each month there is a probability of 0.25 that he will buy shares in a large company, there is a probability of 0.15 that he will buy shares in a small company and there is a probability of 0.6 that he will invest in a savings account.
Find the probability that George will buy shares in a small company in at least 3 of these 18 months.