Join TSR now and get all your revision questions answeredSign up now

Probability Confusion Watch

    • Thread Starter

    Hey all,

    I'm having a bit of trouble understanding probability.

    So I came across this data with a question:

    Name:  2016-10-16 05_11_20-Microsoft Word - Tutorial Week 3-Questions.docx.png
Views: 31
Size:  20.0 KB

    The question is:

    "Calculate the probability that a randomly selected shareholder owns fewer than 200 shares or is against the merger."

    Now I thought that I could just add the 12,3,10 and 6 together to get 31, therefore the probability would be 31/50.

    I then decided that since I knew the answer I would try using the formula: "p(a)+p(b)-p(a∩b)" to get the same answer, however, this was not the case. Instead, I ended up getting 19/50+15/50 - (19/50x15/50) = 283/500

    Obviously, these are different answers so... Which one is correct and where did I go wrong with the other?


    (Also, sorry for making this so long, I wanted this to be fairly detailed so you could see my steps and thought process)

    The problem with the second method is the working out of P(A and B). P(A and B) = P(A) x P(B) is only true for independent events.
What is your favourite film genre?
Useful resources

Make your revision easier


Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here


How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Quick reply
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.