Probability Skill Examples


Lets assume that the student asked the solutions to the below questions.




1.      The admin officer has written five letters to five applicants on the short list giving them interview times for the second stage of selection process. She has earlier typed the addresses on the envelopes. While she was distracted she put the letters in the envelopes in random. Find the probability none of the letters are in the right envelopes.


2.      The probability of a marksman hitting a target is 0.9. What is the number of shots required if the probability of him hitting the target at least once is 0.999?    


You can solve this question by seeing the solutions to the given questions below, which use the same concepts as the student’s questions.



Questions which will help you to solve your question.


Question 1: If a jeweller puts six rings into six boxes at random, what is the probability that all the rings are in correct boxes?




There are six boxes. One out of six rings belong to the first box, and one out of five ring belongs to the second box and so on.















P(all rings are in their correct boxes)First ring in the correct box and second ring in the correct box and . . .

P(all rings are in their correct boxes)


Question 2: Jane attempts the crossword puzzle in the newspaper daily. In the last two years she found that on average she successfully completes three out of four puzzles. In how many days probability of her completing at least one crossword puzzle successfully is 0.98?


Answer:    and

p(at least one)

p(at least one) = 1- p(none)

p(at least one)



Determine the values of p and q in your question.