Probability

Here are few types of questions.

1) A,B,C,D,E are sitting in a row. Find the probability that A and B are sitting together.

Answer : 2/5

Similar method for more people sitting together

Answer : E

Case 1: The ball is put into a non-empty bag. (4/7)
             Pick a ball from an empty bag. (3/7)
Case 2: The ball is put into a empty bag. (3/7)
             Pick a ball from an empty bag. (2/7)

Answer : A

Note that two 2s and two 5s are distinct. Sometimes, counting the combinations is a good choice.










Remarks: 
1) Different people solve the same question by different methods. Some may use nCr and nPr. Some may just count the combinations. Some may multiply the probability of different events. Try different methods for the same question and you will learn more for sure.