Q. What is Addition Rule of probability?
Ans.
Suppose there are 17 girls and 15 boys in your stats class. There are 17 + 15 = 32 ways for your teacher to pick one student to answer a question. Your teacher could either pick a girl or a boy to answer the question, but not both.
In general, suppose we can do job 1 in p ways and job 2 in q ways. Then we can do either job 1 or job 2, but not both, in p + q ways.