概率统计计算

题目

概率统计计算
In a study of 85 third year students,it was found that 30 of them had a job (usually part time),33 had achieved a credit level in the previous year,and 23 had a job AND had achieved a credit level in the previous year.Using a contingency table or otherwise,answer the following questions.
Find the probability that a student,chosen at random from those who achieved credit level,had a job.

答案

有工作的概率是30/85
有信用等级的概率是33/85
既有工作又有信用等级的是23/85
从有信用等级的人中,随机抽出一个有工作的概率是23/33