英文数学概率问题A company takes out an insurance policy to cover acc

英文数学概率问题
A company takes out an insurance policy to cover accidents that occur at its manufacturing plant.The probability that one or more accidents will occur during any given month is 3/5.The number of accidents that occur in any given month is independent of the number of accidents that occue in all other month.Calculate the probability that there wil be at least four months in which no accidencts occur before the fourth month in which at least one accident occurs. 我想求最后一句的翻译